diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/WhisperServerService.java b/service/src/main/java/org/whispersystems/textsecuregcm/WhisperServerService.java
index fbdc1b019..97d47be60 100644
--- a/service/src/main/java/org/whispersystems/textsecuregcm/WhisperServerService.java
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/WhisperServerService.java
@@ -100,6 +100,7 @@ import org.whispersystems.textsecuregcm.mappers.DeviceLimitExceededExceptionMapp
 import org.whispersystems.textsecuregcm.mappers.IOExceptionMapper;
 import org.whispersystems.textsecuregcm.mappers.InvalidWebsocketAddressExceptionMapper;
 import org.whispersystems.textsecuregcm.mappers.RateLimitExceededExceptionMapper;
+import org.whispersystems.textsecuregcm.mappers.RetryLaterExceptionMapper;
 import org.whispersystems.textsecuregcm.metrics.BufferPoolGauges;
 import org.whispersystems.textsecuregcm.metrics.CpuUsageGauge;
 import org.whispersystems.textsecuregcm.metrics.FileDescriptorGauge;
@@ -498,16 +499,19 @@ public class WhisperServerService extends Application<WhisperServerConfiguration
     environment.jersey().register(new RateLimitExceededExceptionMapper());
     environment.jersey().register(new InvalidWebsocketAddressExceptionMapper());
     environment.jersey().register(new DeviceLimitExceededExceptionMapper());
+    environment.jersey().register(new RetryLaterExceptionMapper());
 
     webSocketEnvironment.jersey().register(new IOExceptionMapper());
     webSocketEnvironment.jersey().register(new RateLimitExceededExceptionMapper());
     webSocketEnvironment.jersey().register(new InvalidWebsocketAddressExceptionMapper());
     webSocketEnvironment.jersey().register(new DeviceLimitExceededExceptionMapper());
+    webSocketEnvironment.jersey().register(new RetryLaterExceptionMapper());
 
     provisioningEnvironment.jersey().register(new IOExceptionMapper());
     provisioningEnvironment.jersey().register(new RateLimitExceededExceptionMapper());
     provisioningEnvironment.jersey().register(new InvalidWebsocketAddressExceptionMapper());
     provisioningEnvironment.jersey().register(new DeviceLimitExceededExceptionMapper());
+    provisioningEnvironment.jersey().register(new RetryLaterExceptionMapper());
   }
 
   private void registerCorsFilter(Environment environment) {
diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/controllers/AccountController.java b/service/src/main/java/org/whispersystems/textsecuregcm/controllers/AccountController.java
index 782a92188..501cfb9fd 100644
--- a/service/src/main/java/org/whispersystems/textsecuregcm/controllers/AccountController.java
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/controllers/AccountController.java
@@ -187,7 +187,7 @@ public class AccountController {
                                 @QueryParam("client")           Optional<String> client,
                                 @QueryParam("captcha")          Optional<String> captcha,
                                 @QueryParam("challenge")        Optional<String> pushChallenge)
-      throws RateLimitExceededException
+      throws RateLimitExceededException, RetryLaterException
   {
     if (!Util.isValidNumber(number)) {
       logger.info("Invalid number: " + number);
@@ -217,16 +217,24 @@ public class AccountController {
       return Response.status(402).build();
     }
 
-    switch (transport) {
-      case "sms":
-        rateLimiters.getSmsDestinationLimiter().validate(number);
-        break;
-      case "voice":
-        rateLimiters.getVoiceDestinationLimiter().validate(number);
-        rateLimiters.getVoiceDestinationDailyLimiter().validate(number);
-        break;
-      default:
-        throw new WebApplicationException(Response.status(422).build());
+    try {
+      switch (transport) {
+        case "sms":
+          rateLimiters.getSmsDestinationLimiter().validate(number);
+          break;
+        case "voice":
+          rateLimiters.getVoiceDestinationLimiter().validate(number);
+          rateLimiters.getVoiceDestinationDailyLimiter().validate(number);
+          break;
+        default:
+          throw new WebApplicationException(Response.status(422).build());
+      }
+    } catch (RateLimitExceededException e) {
+      if (!e.getRetryDuration().isNegative()) {
+        throw new RetryLaterException(e);
+      } else {
+        throw e;
+      }
     }
 
     VerificationCode       verificationCode       = generateVerificationCode(number);
diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/controllers/RateLimitExceededException.java b/service/src/main/java/org/whispersystems/textsecuregcm/controllers/RateLimitExceededException.java
index f0daf59ed..9c7e4f1da 100644
--- a/service/src/main/java/org/whispersystems/textsecuregcm/controllers/RateLimitExceededException.java
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/controllers/RateLimitExceededException.java
@@ -4,12 +4,26 @@
  */
 package org.whispersystems.textsecuregcm.controllers;
 
+import java.time.Duration;
+
 public class RateLimitExceededException extends Exception {
+
+  private final Duration retryDuration;
+
   public RateLimitExceededException() {
     super();
+    retryDuration = Duration.ZERO;
   }
 
-  public RateLimitExceededException(String number) {
-    super(number);
+  public RateLimitExceededException(String message) {
+    super(message);
+    retryDuration = Duration.ZERO;
   }
+
+  public RateLimitExceededException(String message, long retryAfterMillis) {
+    super(message);
+    retryDuration = Duration.ofMillis(retryAfterMillis);
+  }
+
+  public Duration getRetryDuration() { return retryDuration; }
 }
diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/controllers/RetryLaterException.java b/service/src/main/java/org/whispersystems/textsecuregcm/controllers/RetryLaterException.java
new file mode 100644
index 000000000..b577aee9d
--- /dev/null
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/controllers/RetryLaterException.java
@@ -0,0 +1,25 @@
+/*
+ * Copyright 2013-2021 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.controllers;
+
+import java.time.Duration;
+
+public class RetryLaterException extends Exception {
+  private final Duration backoffDuration;
+
+  public RetryLaterException() {
+    backoffDuration = Duration.ZERO;
+  }
+  public RetryLaterException(int retryLaterMillis) {
+    backoffDuration = Duration.ofMillis(retryLaterMillis);
+  }
+
+  public RetryLaterException(RateLimitExceededException e) {
+    this.backoffDuration = e.getRetryDuration();
+  }
+
+  public Duration getBackoffDuration() { return backoffDuration; }
+}
diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/limits/LeakyBucket.java b/service/src/main/java/org/whispersystems/textsecuregcm/limits/LeakyBucket.java
index 08e960fdf..6f76b98fd 100644
--- a/service/src/main/java/org/whispersystems/textsecuregcm/limits/LeakyBucket.java
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/limits/LeakyBucket.java
@@ -48,6 +48,18 @@ public class LeakyBucket {
                     (int)Math.floor(this.spaceRemaining + (elapsedTime * this.leakRatePerMillis)));
   }
 
+  public long getMillisUntilSpace(double amount) {
+    int currentSpaceRemaining = getUpdatedSpaceRemaining();
+    if (currentSpaceRemaining >= amount) {
+      return 0;
+    } else if (amount > this.bucketSize) {
+      // This shouldn't happen today but if so we should bubble this to the clients somehow
+      return -1;
+    } else {
+      return (long)Math.ceil(amount - currentSpaceRemaining / this.leakRatePerMillis);
+    }
+  }
+
   public String serialize(ObjectMapper mapper) throws JsonProcessingException {
     return mapper.writeValueAsString(new LeakyBucketEntity(bucketSize, leakRatePerMillis, spaceRemaining, lastUpdateTimeMillis));
   }
diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/limits/RateLimiter.java b/service/src/main/java/org/whispersystems/textsecuregcm/limits/RateLimiter.java
index 03c6a9a9e..8006c40d1 100644
--- a/service/src/main/java/org/whispersystems/textsecuregcm/limits/RateLimiter.java
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/limits/RateLimiter.java
@@ -28,7 +28,7 @@ public class RateLimiter {
   private final ObjectMapper mapper = SystemMapper.getMapper();
 
   private   final Meter                     meter;
-  private   final Timer                     validateTimer;
+  protected final Timer                     validateTimer;
   protected final FaultTolerantRedisCluster cacheCluster;
   protected final String                    name;
   private   final int                       bucketSize;
@@ -66,7 +66,7 @@ public class RateLimiter {
         setBucket(key, bucket);
       } else {
         meter.mark();
-        throw new RateLimitExceededException(key + " , " + amount);
+        throw new RateLimitExceededException(key + " , " + amount, bucket.getMillisUntilSpace(amount));
       }
     }
   }
@@ -87,7 +87,7 @@ public class RateLimiter {
     return leakRatePerMinute;
   }
 
-  private void setBucket(String key, LeakyBucket bucket) {
+  protected void setBucket(String key, LeakyBucket bucket) {
     try {
       final String serialized = bucket.serialize(mapper);
 
@@ -97,7 +97,7 @@ public class RateLimiter {
     }
   }
 
-  private LeakyBucket getBucket(String key) {
+  protected LeakyBucket getBucket(String key) {
     try {
       final String serialized = cacheCluster.withCluster(connection -> connection.sync().get(getBucketName(key)));
 
diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/mappers/RetryLaterExceptionMapper.java b/service/src/main/java/org/whispersystems/textsecuregcm/mappers/RetryLaterExceptionMapper.java
new file mode 100644
index 000000000..7d868df11
--- /dev/null
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/mappers/RetryLaterExceptionMapper.java
@@ -0,0 +1,29 @@
+/*
+ * Copyright 2013-2020 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.mappers;
+
+import org.whispersystems.textsecuregcm.controllers.RateLimitExceededException;
+import javax.ws.rs.core.Response;
+import javax.ws.rs.ext.ExceptionMapper;
+import javax.ws.rs.ext.Provider;
+
+import org.whispersystems.textsecuregcm.controllers.RetryLaterException;
+
+import javax.ws.rs.core.Response;
+import javax.ws.rs.ext.ExceptionMapper;
+import javax.ws.rs.ext.Provider;
+import java.time.Duration;
+
+@Provider
+public class RetryLaterExceptionMapper implements ExceptionMapper<RetryLaterException> {
+  @Override
+  public Response toResponse(RetryLaterException e) {
+    return Response.status(413)
+                   .header("Retry-After", e.getBackoffDuration().toSeconds())
+                   .build();
+  }
+}
+