diff --git a/src/main/java/life/mosu/mosuserver/application/auth/processor/SignUpAccountStepProcessor.java b/src/main/java/life/mosu/mosuserver/application/auth/processor/SignUpAccountStepProcessor.java
index bab8cb99..6adfd03c 100644
--- a/src/main/java/life/mosu/mosuserver/application/auth/processor/SignUpAccountStepProcessor.java
+++ b/src/main/java/life/mosu/mosuserver/application/auth/processor/SignUpAccountStepProcessor.java
@@ -18,7 +18,8 @@ public class SignUpAccountStepProcessor implements StepProcessor<UserJpaEntity,
     @Transactional
     @Override
     public UserJpaEntity process(UserJpaEntity user) {
-        if (userRepository.existsByLoginId(user.getLoginId())) {
+        if (userRepository.existsByPhoneNumber(user.getPhoneNumber())
+                || userRepository.existsByLoginId(user.getLoginId())) {
             throw new CustomRuntimeException(ErrorCode.USER_ALREADY_EXISTS);
         }
         return userRepository.save(user);
diff --git a/src/main/java/life/mosu/mosuserver/application/oauth/OAuthUserPersistenceProcessor.java b/src/main/java/life/mosu/mosuserver/application/oauth/OAuthUserPersistenceProcessor.java
index 3e823364..76e46921 100644
--- a/src/main/java/life/mosu/mosuserver/application/oauth/OAuthUserPersistenceProcessor.java
+++ b/src/main/java/life/mosu/mosuserver/application/oauth/OAuthUserPersistenceProcessor.java
@@ -18,7 +18,7 @@ public class OAuthUserPersistenceProcessor implements StepProcessor<OAuthUserInf
     @Override
     @Transactional
     public UserJpaEntity process(final OAuthUserInfo info) {
-        return userRepository.findByLoginId(info.email())
+        return userRepository.findByPhoneNumber(info.phoneNumber())
                 .map(existingUser -> {
                     existingUser.updateOAuthUser(
                             info.gender(),
diff --git a/src/main/java/life/mosu/mosuserver/application/refund/RefundService.java b/src/main/java/life/mosu/mosuserver/application/refund/RefundService.java
index ffd8d406..2cbbff09 100644
--- a/src/main/java/life/mosu/mosuserver/application/refund/RefundService.java
+++ b/src/main/java/life/mosu/mosuserver/application/refund/RefundService.java
@@ -10,10 +10,12 @@
 import life.mosu.mosuserver.global.exception.ErrorCode;
 import life.mosu.mosuserver.infra.toss.dto.CancelTossPaymentResponse;
 import life.mosu.mosuserver.presentation.refund.dto.MergedRefundRequest;
+import life.mosu.mosuserver.presentation.refund.dto.RefundAmountResponse;
 import life.mosu.mosuserver.presentation.refund.dto.RefundRequest;
 import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
 import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Propagation;
 import org.springframework.transaction.annotation.Transactional;
 
 
@@ -28,6 +30,16 @@ public class RefundService {
     private final TossRefundProcessor tossRefundProcessor;
     private final PaymentJpaRepository paymentJpaRepository;
 
+    @Transactional(propagation = Propagation.SUPPORTS, readOnly = true)
+    public RefundAmountResponse getRefundAmount(String paymentKey, Long examApplicationId) {
+        PaymentWithLunchProjection targetPayment = findPaymentOrThrow(paymentKey,
+                examApplicationId);
+
+        int totalQuantity = getTotalPaymentCount(paymentKey);
+        int refundAmount = calculateRefundAmount(totalQuantity, targetPayment.lunchChecked());
+        return RefundAmountResponse.of(refundAmount);
+    }
+
     @Transactional
     public void doProcess(Long userId, MergedRefundRequest request) {
         RefundRequest details = request.details();
diff --git a/src/main/java/life/mosu/mosuserver/application/user/UserService.java b/src/main/java/life/mosu/mosuserver/application/user/UserService.java
index 39c9a663..be991f70 100644
--- a/src/main/java/life/mosu/mosuserver/application/user/UserService.java
+++ b/src/main/java/life/mosu/mosuserver/application/user/UserService.java
@@ -51,4 +51,8 @@ public UserInfoResponse getUserInfo(Long userId) {
 
         return UserInfoResponse.from(user);
     }
+
+    public Boolean isPhoneNumberAvailable(String phoneNumber) {
+        return !userJpaRepository.existsByPhoneNumber(phoneNumber);
+    }
 }
diff --git a/src/main/java/life/mosu/mosuserver/domain/user/repository/UserJpaRepository.java b/src/main/java/life/mosu/mosuserver/domain/user/repository/UserJpaRepository.java
index cd553965..e5038a33 100644
--- a/src/main/java/life/mosu/mosuserver/domain/user/repository/UserJpaRepository.java
+++ b/src/main/java/life/mosu/mosuserver/domain/user/repository/UserJpaRepository.java
@@ -10,8 +10,9 @@ public interface UserJpaRepository extends JpaRepository<UserJpaEntity, Long> {
 
     boolean existsByLoginId(String loginId);
 
-
     Optional<UserJpaEntity> findByNameAndPhoneNumber(String name, String phoneNumber);
 
     Optional<UserJpaEntity> findByPhoneNumber(String phoneNumber);
+
+    boolean existsByPhoneNumber(String phoneNumber);
 }
diff --git a/src/main/java/life/mosu/mosuserver/global/filter/Whitelist.java b/src/main/java/life/mosu/mosuserver/global/filter/Whitelist.java
index 47e65e9e..ea197fdb 100644
--- a/src/main/java/life/mosu/mosuserver/global/filter/Whitelist.java
+++ b/src/main/java/life/mosu/mosuserver/global/filter/Whitelist.java
@@ -42,6 +42,9 @@ public enum Whitelist {
     FAQ("/api/v1/faq", WhitelistMethod.GET),
     NOTICE("/api/v1/notice", WhitelistMethod.GET),
     USER_ID_CHECK("/api/v1/user/check-id", WhitelistMethod.GET),
+    USER_PHONE_NUMBER_CHECK("/api/v1/user/check-phone-number", WhitelistMethod.GET),
+
+    CUSTOMER_KEY_CHECK("/api/v1/user/customer-key", WhitelistMethod.GET),
     EXAM("/api/v1/exam", WhitelistMethod.GET),
     EXAM_AREAS("/api/v1/exam/areas", WhitelistMethod.GET),
     EXAM_ALL("/api/v1/exam/all", WhitelistMethod.GET),
diff --git a/src/main/java/life/mosu/mosuserver/presentation/refund/RefundController.java b/src/main/java/life/mosu/mosuserver/presentation/refund/RefundController.java
index 85688606..ce09a8e4 100644
--- a/src/main/java/life/mosu/mosuserver/presentation/refund/RefundController.java
+++ b/src/main/java/life/mosu/mosuserver/presentation/refund/RefundController.java
@@ -4,16 +4,19 @@
 import life.mosu.mosuserver.global.annotation.UserId;
 import life.mosu.mosuserver.global.util.ApiResponseWrapper;
 import life.mosu.mosuserver.presentation.refund.dto.MergedRefundRequest;
+import life.mosu.mosuserver.presentation.refund.dto.RefundAmountResponse;
 import life.mosu.mosuserver.presentation.refund.dto.RefundRequest;
 import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
 import org.springframework.security.access.prepost.PreAuthorize;
+import org.springframework.web.bind.annotation.GetMapping;
 import org.springframework.web.bind.annotation.PathVariable;
 import org.springframework.web.bind.annotation.PostMapping;
 import org.springframework.web.bind.annotation.RequestBody;
 import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RequestParam;
 import org.springframework.web.bind.annotation.RestController;
 
 @RestController
@@ -24,6 +27,15 @@ public class RefundController {
 
     private final RefundService refundService;
 
+    @GetMapping()
+    ResponseEntity<ApiResponseWrapper<RefundAmountResponse>> getRefundAmount(
+            @RequestParam(required = true) String paymentKey,
+            @RequestParam(required = true) Long examApplicationId) {
+        var response = refundService.getRefundAmount(paymentKey, examApplicationId);
+        return ResponseEntity.ok(
+                ApiResponseWrapper.success(HttpStatus.OK, "환불 금액 조회 성공", response));
+    }
+
     @PostMapping("/{paymentKey}")
     @PreAuthorize("isAuthenticated() and hasRole('USER')")
     ResponseEntity<ApiResponseWrapper<Void>> process(
diff --git a/src/main/java/life/mosu/mosuserver/presentation/refund/dto/RefundAmountResponse.java b/src/main/java/life/mosu/mosuserver/presentation/refund/dto/RefundAmountResponse.java
new file mode 100644
index 00000000..2cfd00f8
--- /dev/null
+++ b/src/main/java/life/mosu/mosuserver/presentation/refund/dto/RefundAmountResponse.java
@@ -0,0 +1,11 @@
+package life.mosu.mosuserver.presentation.refund.dto;
+
+public record RefundAmountResponse(
+        int amount
+) {
+
+    public static RefundAmountResponse of(int amount) {
+        return new RefundAmountResponse(amount);
+    }
+
+}
diff --git a/src/main/java/life/mosu/mosuserver/presentation/user/UserController.java b/src/main/java/life/mosu/mosuserver/presentation/user/UserController.java
index 0a5f0bf3..016bb936 100644
--- a/src/main/java/life/mosu/mosuserver/presentation/user/UserController.java
+++ b/src/main/java/life/mosu/mosuserver/presentation/user/UserController.java
@@ -2,10 +2,12 @@
 
 import life.mosu.mosuserver.application.user.UserService;
 import life.mosu.mosuserver.global.annotation.LoginIdPattern;
+import life.mosu.mosuserver.global.annotation.PhoneNumberPattern;
 import life.mosu.mosuserver.global.annotation.UserId;
 import life.mosu.mosuserver.global.util.ApiResponseWrapper;
-import life.mosu.mosuserver.presentation.user.dto.request.IsLoginIdAvailableResponse;
 import life.mosu.mosuserver.presentation.user.dto.response.CustomerKeyResponse;
+import life.mosu.mosuserver.presentation.user.dto.response.IsLoginIdAvailableResponse;
+import life.mosu.mosuserver.presentation.user.dto.response.IsPhoneNumberAvailableResponse;
 import life.mosu.mosuserver.presentation.user.dto.response.UserInfoResponse;
 import lombok.RequiredArgsConstructor;
 import org.springframework.http.HttpStatus;
@@ -58,4 +60,14 @@ public ResponseEntity<ApiResponseWrapper<IsLoginIdAvailableResponse>> isLoginIdA
                         IsLoginIdAvailableResponse.from(isLoginIdAvailable)));
     }
 
+    @GetMapping("/check-phone-number")
+    public ResponseEntity<ApiResponseWrapper<IsPhoneNumberAvailableResponse>> isPhoneNumberAvailable(
+            @PhoneNumberPattern @RequestParam String phoneNumber
+    ) {
+        Boolean isPhoneNumberAvailable = userService.isPhoneNumberAvailable(phoneNumber);
+
+        return ResponseEntity.ok(
+                ApiResponseWrapper.success(HttpStatus.OK, "User PhoneNumber 등록 가능 여부 조회 성공",
+                        IsPhoneNumberAvailableResponse.from(isPhoneNumberAvailable)));
+    }
 }
diff --git a/src/main/java/life/mosu/mosuserver/presentation/user/UserControllerDocs.java b/src/main/java/life/mosu/mosuserver/presentation/user/UserControllerDocs.java
index c766bb29..64fc6785 100644
--- a/src/main/java/life/mosu/mosuserver/presentation/user/UserControllerDocs.java
+++ b/src/main/java/life/mosu/mosuserver/presentation/user/UserControllerDocs.java
@@ -10,8 +10,8 @@
 import life.mosu.mosuserver.global.annotation.LoginIdPattern;
 import life.mosu.mosuserver.global.annotation.UserId;
 import life.mosu.mosuserver.global.util.ApiResponseWrapper;
-import life.mosu.mosuserver.presentation.user.dto.request.IsLoginIdAvailableResponse;
 import life.mosu.mosuserver.presentation.user.dto.response.CustomerKeyResponse;
+import life.mosu.mosuserver.presentation.user.dto.response.IsLoginIdAvailableResponse;
 import org.springframework.http.ResponseEntity;
 import org.springframework.validation.annotation.Validated;
 import org.springframework.web.bind.annotation.RequestParam;
diff --git a/src/main/java/life/mosu/mosuserver/presentation/user/dto/request/IsLoginIdAvailableResponse.java b/src/main/java/life/mosu/mosuserver/presentation/user/dto/response/IsLoginIdAvailableResponse.java
similarity index 79%
rename from src/main/java/life/mosu/mosuserver/presentation/user/dto/request/IsLoginIdAvailableResponse.java
rename to src/main/java/life/mosu/mosuserver/presentation/user/dto/response/IsLoginIdAvailableResponse.java
index 81854a7f..48a0aa70 100644
--- a/src/main/java/life/mosu/mosuserver/presentation/user/dto/request/IsLoginIdAvailableResponse.java
+++ b/src/main/java/life/mosu/mosuserver/presentation/user/dto/response/IsLoginIdAvailableResponse.java
@@ -1,4 +1,4 @@
-package life.mosu.mosuserver.presentation.user.dto.request;
+package life.mosu.mosuserver.presentation.user.dto.response;
 
 public record IsLoginIdAvailableResponse(
         Boolean isLoginIdAvailable
diff --git a/src/main/java/life/mosu/mosuserver/presentation/user/dto/response/IsPhoneNumberAvailableResponse.java b/src/main/java/life/mosu/mosuserver/presentation/user/dto/response/IsPhoneNumberAvailableResponse.java
new file mode 100644
index 00000000..91a5ff05
--- /dev/null
+++ b/src/main/java/life/mosu/mosuserver/presentation/user/dto/response/IsPhoneNumberAvailableResponse.java
@@ -0,0 +1,10 @@
+package life.mosu.mosuserver.presentation.user.dto.response;
+
+public record IsPhoneNumberAvailableResponse(
+        Boolean isPhoneNumberAvailable
+) {
+
+    public static IsPhoneNumberAvailableResponse from(Boolean isPhoneNumberAvailable) {
+        return new IsPhoneNumberAvailableResponse(isPhoneNumberAvailable);
+    }
+}