diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/util/ImpossibleNumberException.java b/service/src/main/java/org/whispersystems/textsecuregcm/util/ImpossibleNumberException.java
new file mode 100644
index 000000000..a07ffe714
--- /dev/null
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/util/ImpossibleNumberException.java
@@ -0,0 +1,17 @@
+/*
+ * Copyright 2013-2021 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.util;
+
+public class ImpossibleNumberException extends Exception {
+
+  public ImpossibleNumberException() {
+    super();
+  }
+
+  public ImpossibleNumberException(final Throwable cause) {
+    super(cause);
+  }
+}
diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/util/NonNormalizedNumberException.java b/service/src/main/java/org/whispersystems/textsecuregcm/util/NonNormalizedNumberException.java
new file mode 100644
index 000000000..c7e821097
--- /dev/null
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/util/NonNormalizedNumberException.java
@@ -0,0 +1,25 @@
+/*
+ * Copyright 2013-2021 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.util;
+
+public class NonNormalizedNumberException extends Exception {
+
+  private final String originalNumber;
+  private final String normalizedNumber;
+
+  public NonNormalizedNumberException(final String originalNumber, final String normalizedNumber) {
+    this.originalNumber = originalNumber;
+    this.normalizedNumber = normalizedNumber;
+  }
+
+  public String getOriginalNumber() {
+    return originalNumber;
+  }
+
+  public String getNormalizedNumber() {
+    return normalizedNumber;
+  }
+}
diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/util/Util.java b/service/src/main/java/org/whispersystems/textsecuregcm/util/Util.java
index 1f67bb3cf..02b0d6e51 100644
--- a/service/src/main/java/org/whispersystems/textsecuregcm/util/Util.java
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/util/Util.java
@@ -4,7 +4,10 @@
  */
 package org.whispersystems.textsecuregcm.util;
 
+import com.google.i18n.phonenumbers.NumberParseException;
 import com.google.i18n.phonenumbers.PhoneNumberUtil;
+import com.google.i18n.phonenumbers.PhoneNumberUtil.PhoneNumberFormat;
+import com.google.i18n.phonenumbers.Phonenumber.PhoneNumber;
 import java.io.UnsupportedEncodingException;
 import java.net.URLEncoder;
 import java.security.MessageDigest;
@@ -28,6 +31,8 @@ public class Util {
 
   private static final Pattern COUNTRY_CODE_PATTERN = Pattern.compile("^\\+([17]|2[07]|3[0123469]|4[013456789]|5[12345678]|6[0123456]|8[1246]|9[0123458]|\\d{3})");
 
+  private static final PhoneNumberUtil PHONE_NUMBER_UTIL = PhoneNumberUtil.getInstance();
+
   public static byte[] getContactToken(String number) {
     try {
       MessageDigest digest    = MessageDigest.getInstance("SHA1");
@@ -41,7 +46,32 @@ public class Util {
   }
 
   public static boolean isValidNumber(String number) {
-    return number.matches("^\\+[0-9]+") && PhoneNumberUtil.getInstance().isPossibleNumber(number, null);
+    return number.matches("^\\+[0-9]+") && PHONE_NUMBER_UTIL.isPossibleNumber(number, null);
+  }
+
+  /**
+   * Checks that the given number is a valid, E164-normalized phone number.
+   *
+   * @param number the number to check
+   *
+   * @throws ImpossibleNumberException if the given number is not a valid phone number at all
+   * @throws NonNormalizedNumberException if the given number is a valid phone number, but isn't E164-normalized
+   */
+  public static void requireNormalizedNumber(final String number) throws ImpossibleNumberException, NonNormalizedNumberException {
+    if (!PHONE_NUMBER_UTIL.isPossibleNumber(number, null)) {
+      throw new ImpossibleNumberException();
+    }
+
+    try {
+      final PhoneNumber phoneNumber = PHONE_NUMBER_UTIL.parse(number, null);
+      final String normalizedNumber = PHONE_NUMBER_UTIL.format(phoneNumber, PhoneNumberFormat.E164);
+
+      if (!number.equals(normalizedNumber)) {
+        throw new NonNormalizedNumberException(number, normalizedNumber);
+      }
+    } catch (final NumberParseException e) {
+      throw new ImpossibleNumberException(e);
+    }
   }
 
   public static String getCountryCode(String number) {
diff --git a/service/src/test/java/org/whispersystems/textsecuregcm/tests/util/ValidNumberTest.java b/service/src/test/java/org/whispersystems/textsecuregcm/tests/util/ValidNumberTest.java
index 1c3366e8a..7f07c727c 100644
--- a/service/src/test/java/org/whispersystems/textsecuregcm/tests/util/ValidNumberTest.java
+++ b/service/src/test/java/org/whispersystems/textsecuregcm/tests/util/ValidNumberTest.java
@@ -5,14 +5,20 @@
 
 package org.whispersystems.textsecuregcm.tests.util;
 
+import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.Arguments;
 import org.junit.jupiter.params.provider.MethodSource;
+import org.junit.jupiter.params.provider.ValueSource;
+import org.whispersystems.textsecuregcm.util.ImpossibleNumberException;
+import org.whispersystems.textsecuregcm.util.NonNormalizedNumberException;
 import org.whispersystems.textsecuregcm.util.Util;
 
 import java.util.stream.Stream;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 class ValidNumberTest {
 
@@ -53,4 +59,51 @@ class ValidNumberTest {
         Arguments.of("+689123456", true)
     );
   }
+
+  @ParameterizedTest
+  @ValueSource(strings = {
+      "+447700900111",
+      "+14151231234",
+      "+71234567890",
+      "+447535742222",
+      "+4915174108888",
+      "+298123456",
+      "+299123456",
+      "+376123456",
+      "+68512345",
+      "+689123456"})
+  void requireNormalizedNumber(final String number) {
+    assertDoesNotThrow(() -> Util.requireNormalizedNumber(number));
+  }
+
+  @Test
+  void requireNormalizedNumberNull() {
+    assertThrows(ImpossibleNumberException.class, () -> Util.requireNormalizedNumber(null));
+  }
+
+  @ParameterizedTest
+  @ValueSource(strings = {
+      "Definitely not a phone number at all",
+      "+141512312341",
+      "+712345678901",
+      "+4475357422221",
+      "+491517410888811111",
+      "71234567890",
+      "001447535742222",
+      "+1415123123a"
+  })
+  void requireNormalizedNumberImpossibleNumber(final String number) {
+    assertThrows(ImpossibleNumberException.class, () -> Util.requireNormalizedNumber(number));
+  }
+
+  @ParameterizedTest
+  @ValueSource(strings = {
+      "+4407700900111",
+      "+1 415 123 1234",
+      "+1 (415) 123-1234",
+      "+1 415)123-1234",
+      " +14151231234"})
+  void requireNormalizedNumberNonNormalized(final String number) {
+    assertThrows(NonNormalizedNumberException.class, () -> Util.requireNormalizedNumber(number));
+  }
 }