diff --git a/changelog.d/6079.feature b/changelog.d/6079.feature
new file mode 100644
index 000000000000..bcbb49ac58a1
--- /dev/null
+++ b/changelog.d/6079.feature
@@ -0,0 +1 @@
+Add `submit_url` response parameter to `*/msisdn/requestToken` endpoints.
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index 61d9f09a9966..10d562b4d254 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -937,6 +937,8 @@ uploads_path: "DATADIR/uploads"
 # by the Matrix Identity Service API specification:
 # https://matrix.org/docs/spec/identity_service/latest
 #
+# If a delegate is specified, the config option public_baseurl must also be filled out.
+#
 account_threepid_delegates:
     #email: https://example.com     # Delegate email sending to example.org
     #msisdn: http://localhost:8090  # Delegate SMS sending to this local process
diff --git a/synapse/config/registration.py b/synapse/config/registration.py
index d4654e99b34a..bef89e2bf4c4 100644
--- a/synapse/config/registration.py
+++ b/synapse/config/registration.py
@@ -293,6 +293,8 @@ def generate_config_section(self, generate_secrets=False, **kwargs):
         # by the Matrix Identity Service API specification:
         # https://matrix.org/docs/spec/identity_service/latest
         #
+        # If a delegate is specified, the config option public_baseurl must also be filled out.
+        #
         account_threepid_delegates:
             #email: https://example.com     # Delegate email sending to example.org
             #msisdn: http://localhost:8090  # Delegate SMS sending to this local process
diff --git a/synapse/handlers/identity.py b/synapse/handlers/identity.py
index af6f5919428d..a1ba370c9791 100644
--- a/synapse/handlers/identity.py
+++ b/synapse/handlers/identity.py
@@ -440,13 +440,23 @@ def requestMsisdnToken(
                 id_server + "/_matrix/identity/api/v1/validate/msisdn/requestToken",
                 params,
             )
-            return data
         except HttpResponseException as e:
             logger.info("Proxied requestToken failed: %r", e)
             raise e.to_synapse_error()
         except TimeoutError:
             raise SynapseError(500, "Timed out contacting identity server")
 
+        assert self.hs.config.public_baseurl
+
+        # we need to tell the client to send the token back to us, since it doesn't
+        # otherwise know where to send it, so add submit_url response parameter
+        # (see also MSC2078)
+        data["submit_url"] = (
+            self.hs.config.public_baseurl
+            + "_matrix/client/unstable/add_threepid/msisdn/submit_token"
+        )
+        return data
+
     @defer.inlineCallbacks
     def validate_threepid_session(self, client_secret, sid):
         """Validates a threepid session with only the client secret and session ID