diff --git a/README.md b/README.md
index 477d2a8..73cdfc4 100644
--- a/README.md
+++ b/README.md
@@ -101,7 +101,7 @@ asyncio.run(
 import asyncio
 from pathlib import Path
 
-from kalyke import VoIPApnsConfig, ApnsPushType, VoIPClient
+from kalyke import VoIPApnsConfig, VoIPClient
 
 client = VoIPClient(
     use_sandbox=True,
diff --git a/examples/voip.py b/examples/voip.py
index 3a75e94..ae31d62 100644
--- a/examples/voip.py
+++ b/examples/voip.py
@@ -1,7 +1,7 @@
 import asyncio
 from pathlib import Path
 
-from kalyke import ApnsConfig, ApnsPushType, VoIPClient
+from kalyke import VoIPApnsConfig, VoIPClient
 
 client = VoIPClient(
     use_sandbox=True,
@@ -9,7 +9,7 @@
 )
 
 payload = {"key": "value"}
-config = ApnsConfig(topic="com.example.App.voip", push_type=ApnsPushType.VOIP)
+config = VoIPApnsConfig(topic="com.example.App.voip")
 
 # Send single VoIP notification
 
diff --git a/kalyke/clients/__init__.py b/kalyke/clients/__init__.py
index aa6bb26..f17a938 100644
--- a/kalyke/clients/__init__.py
+++ b/kalyke/clients/__init__.py
@@ -1,5 +1,6 @@
 import importlib
 import urllib.parse
+from pathlib import Path
 from typing import Any, Dict, Union
 
 from httpx import AsyncClient, Response
@@ -10,6 +11,7 @@
 
 class __Client(object):
     use_sandbox: bool
+    auth_key_filepath: Union[str, Path]
 
     async def send_message(
         self,
@@ -55,3 +57,9 @@ def _handle_error(self, error_json: Dict[str, Any]) -> ApnsProviderException:
         exception_class = getattr(exceptions_module, reason)
 
         return exception_class(error=error_json)
+
+    def _get_auth_key_filepath(self) -> Path:
+        if isinstance(self.auth_key_filepath, Path):
+            return self.auth_key_filepath
+        else:
+            return Path(self.auth_key_filepath)
diff --git a/kalyke/clients/apns.py b/kalyke/clients/apns.py
index 560de3d..73947fd 100644
--- a/kalyke/clients/apns.py
+++ b/kalyke/clients/apns.py
@@ -10,19 +10,12 @@
 from . import __Client as BaseClient
 
 
-@dataclass
+@dataclass(frozen=True)
 class ApnsClient(BaseClient):
     use_sandbox: bool
     team_id: str
     auth_key_id: str
     auth_key_filepath: Union[str, Path]
-    _auth_key_filepath: Path = field(init=False)
-
-    def __post_init__(self):
-        if isinstance(self.auth_key_filepath, Path):
-            self._auth_key_filepath = self.auth_key_filepath
-        else:
-            self._auth_key_filepath = Path(self.auth_key_filepath)
 
     def _init_client(self, apns_config: ApnsConfig) -> AsyncClient:
         headers = apns_config.make_headers()
@@ -31,7 +24,7 @@ def _init_client(self, apns_config: ApnsConfig) -> AsyncClient:
         return client
 
     def _make_authorization(self) -> str:
-        auth_key = self._auth_key_filepath.read_text()
+        auth_key = self._get_auth_key_filepath().read_text()
         token = jwt.encode(
             payload={
                 "iss": self.team_id,
diff --git a/kalyke/clients/live_activity.py b/kalyke/clients/live_activity.py
index 288a18a..cf406d7 100644
--- a/kalyke/clients/live_activity.py
+++ b/kalyke/clients/live_activity.py
@@ -7,7 +7,7 @@
 from .apns import ApnsClient
 
 
-@dataclass
+@dataclass(frozen=True)
 class LiveActivityClient(ApnsClient):
     async def send_message(
         self,
diff --git a/kalyke/clients/voip.py b/kalyke/clients/voip.py
index aaa4a56..c892932 100644
--- a/kalyke/clients/voip.py
+++ b/kalyke/clients/voip.py
@@ -1,6 +1,7 @@
+import warnings
 from dataclasses import dataclass, field
 from pathlib import Path
-from typing import Any, Dict, Union
+from typing import Any, Dict, Optional, Union
 
 import httpx
 from httpx import AsyncClient
@@ -9,17 +10,16 @@
 from . import __Client as BaseClient
 
 
-@dataclass
+@dataclass(frozen=True)
 class VoIPClient(BaseClient):
     use_sandbox: bool
     auth_key_filepath: Union[str, Path]
-    _auth_key_filepath: Path = field(init=False)
+    key_filepath: Optional[Union[str, Path]] = field(default=None)
+    password: Optional[str] = field(default=None)
 
-    def __post_init__(self):
-        if isinstance(self.auth_key_filepath, Path):
-            self._auth_key_filepath = self.auth_key_filepath
-        else:
-            self._auth_key_filepath = Path(self.auth_key_filepath)
+    def __post_init__(self) -> None:
+        if self.key_filepath is None and self.password is not None:
+            warnings.warn(UserWarning("password is ignored because key_filepath is None."))
 
     async def send_message(
         self,
@@ -36,6 +36,10 @@ async def send_message(
     def _init_client(self, apns_config: VoIPApnsConfig) -> AsyncClient:
         headers = apns_config.make_headers()
         context = httpx.create_ssl_context()
-        context.load_cert_chain(self._auth_key_filepath)
+        context.load_cert_chain(
+            certfile=self._get_auth_key_filepath(),
+            keyfile=self.key_filepath,
+            password=self.password,
+        )
         client = AsyncClient(headers=headers, verify=context, http2=True)
         return client
diff --git a/poetry.lock b/poetry.lock
index e3a7344..e33e885 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -666,6 +666,23 @@ files = [
     {file = "pathspec-0.12.1.tar.gz", hash = "sha256:a482d51503a1ab33b1c67a6c3813a26953dbdc71c31dacaef9a838c4e29f5712"},
 ]
 
+[[package]]
+name = "pem"
+version = "23.1.0"
+description = "PEM file parsing in Python."
+optional = false
+python-versions = ">=3.8"
+files = [
+    {file = "pem-23.1.0-py3-none-any.whl", hash = "sha256:78bbb1e75b737891350cb9499cbba31da5d59545f360f44163c0bc751cad55d3"},
+    {file = "pem-23.1.0.tar.gz", hash = "sha256:06503ff2441a111f853ce4e8b9eb9d5fedb488ebdbf560115d3dd53a1b4afc73"},
+]
+
+[package.extras]
+dev = ["pem[tests,types]", "twisted[tls]"]
+docs = ["furo", "myst-parser", "sphinx", "sphinx-notfound-page", "twisted[tls]"]
+tests = ["certifi", "coverage[toml] (>=5.0.2)", "pretend", "pyopenssl", "pytest"]
+types = ["mypy", "twisted", "types-pyopenssl"]
+
 [[package]]
 name = "platformdirs"
 version = "4.1.0"
@@ -895,4 +912,4 @@ zstd = ["zstandard (>=0.18.0)"]
 [metadata]
 lock-version = "2.0"
 python-versions = "^3.9"
-content-hash = "99a037374423c55da87d91dda0c710b971ec28d4577cc448185ad94aa28ca681"
+content-hash = "d380fdabb162afc955917a2148e41445fd926117adb6a2fd86659c909bd868ed"
diff --git a/pyproject.toml b/pyproject.toml
index 1a9db5f..4209ee0 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -40,6 +40,7 @@ pytest-cov = "^4.0.0"
 coveralls = "^3.3.1"
 pytest-httpx = ">=0.21.3,<0.29.0"
 pytest-asyncio = ">=0.20.3,<0.24.0"
+pem = "^23.1.0"
 
 [tool.poetry-dynamic-versioning]
 enable = true
diff --git a/tests/clients/client/test_get_auth_key_filepath.py b/tests/clients/client/test_get_auth_key_filepath.py
new file mode 100644
index 0000000..5912128
--- /dev/null
+++ b/tests/clients/client/test_get_auth_key_filepath.py
@@ -0,0 +1,22 @@
+from pathlib import Path
+
+import pytest
+
+from kalyke.clients import __Client
+
+
+@pytest.mark.parametrize(
+    "auth_key_filepath",
+    [
+        Path(__file__).parent.parent / "dummy.p8",
+        "../dummy.p8",
+    ],
+    ids=["Path object", "str"],
+)
+def test_get_auth_key_filepath(auth_key_filepath):
+    client = __Client()
+    client.auth_key_filepath = auth_key_filepath
+
+    actual = client._get_auth_key_filepath()
+
+    assert isinstance(actual, Path)
diff --git a/tests/clients/voip/test_init.py b/tests/clients/voip/test_init.py
index 6970ffd..25ec68a 100644
--- a/tests/clients/voip/test_init.py
+++ b/tests/clients/voip/test_init.py
@@ -1,3 +1,9 @@
+import tempfile
+from pathlib import Path
+
+import pem
+import pytest
+
 from kalyke import VoIPClient
 
 
@@ -17,3 +23,51 @@ def test_initialize_with_str(auth_key_filepath):
     )
 
     assert isinstance(client, VoIPClient)
+
+
+def test_initialize_with_key_filepath(auth_key_filepath):
+    cer, key = pem.parse_file(auth_key_filepath)
+    with tempfile.TemporaryDirectory() as tmpdir:
+        tmpdir_path = Path(tmpdir)
+        cer_path = tmpdir_path / "cer.pem"
+        cer_path.write_bytes(cer.as_bytes())
+        key_path = tmpdir_path / "key.pem"
+        key_path.write_bytes(key.as_bytes())
+
+        client = VoIPClient(
+            use_sandbox=True,
+            auth_key_filepath=cer_path,
+            key_filepath=key_path,
+        )
+
+        assert isinstance(client, VoIPClient)
+
+
+def test_initialize_with_key_filepath_and_password(auth_key_filepath):
+    cer, key = pem.parse_file(auth_key_filepath)
+    with tempfile.TemporaryDirectory() as tmpdir:
+        tmpdir_path = Path(tmpdir)
+        cer_path = tmpdir_path / "cer.pem"
+        cer_path.write_bytes(cer.as_bytes())
+        key_path = tmpdir_path / "key.pem"
+        key_path.write_bytes(key.as_bytes())
+
+        client = VoIPClient(
+            use_sandbox=True,
+            auth_key_filepath=cer_path,
+            key_filepath=key_path,
+            password="password",
+        )
+
+    assert isinstance(client, VoIPClient)
+
+
+def test_initialize_with_password(auth_key_filepath):
+    with pytest.warns(UserWarning, match="password is ignored because key_filepath is None."):
+        client = VoIPClient(
+            use_sandbox=True,
+            auth_key_filepath=auth_key_filepath,
+            password="password",
+        )
+
+        assert isinstance(client, VoIPClient)