diff --git a/src/PhoneNumber.php b/src/PhoneNumber.php
index 396bbac..002cd72 100644
--- a/src/PhoneNumber.php
+++ b/src/PhoneNumber.php
@@ -11,7 +11,7 @@
 /**
  * A phone number.
  */
-class PhoneNumber implements JsonSerializable
+final class PhoneNumber implements JsonSerializable
 {
     /**
      * The underlying PhoneNumber object from libphonenumber.
diff --git a/src/PhoneNumberFormat.php b/src/PhoneNumberFormat.php
index cea4033..00f9480 100644
--- a/src/PhoneNumberFormat.php
+++ b/src/PhoneNumberFormat.php
@@ -7,7 +7,7 @@
 /**
  * Constants for the phone number formats.
  */
-class PhoneNumberFormat
+final class PhoneNumberFormat
 {
     /**
      * The E164 format.
diff --git a/src/PhoneNumberParseException.php b/src/PhoneNumberParseException.php
index 76493eb..7d5dcdc 100644
--- a/src/PhoneNumberParseException.php
+++ b/src/PhoneNumberParseException.php
@@ -7,7 +7,7 @@
 /**
  * Exception thrown when a phone number cannot be parsed.
  */
-class PhoneNumberParseException extends PhoneNumberException
+final class PhoneNumberParseException extends PhoneNumberException
 {
     /**
      * @internal
diff --git a/src/PhoneNumberType.php b/src/PhoneNumberType.php
index 5db4332..228a0cf 100644
--- a/src/PhoneNumberType.php
+++ b/src/PhoneNumberType.php
@@ -7,7 +7,7 @@
 /**
  * Constants for the phone number types.
  */
-class PhoneNumberType
+final class PhoneNumberType
 {
     /**
      * Fixed line number.