diff --git a/changelog.d/4423.feature b/changelog.d/4423.feature
new file mode 100644
index 000000000..74aeab6d3
--- /dev/null
+++ b/changelog.d/4423.feature
@@ -0,0 +1 @@
+Config option to disable requesting MSISDN on registration.
diff --git a/synapse/config/registration.py b/synapse/config/registration.py
index 6c2b543b8..fe520d685 100644
--- a/synapse/config/registration.py
+++ b/synapse/config/registration.py
@@ -50,6 +50,10 @@ class RegistrationConfig(Config):
                 raise ConfigError('Invalid auto_join_rooms entry %s' % (room_alias,))
         self.autocreate_auto_join_rooms = config.get("autocreate_auto_join_rooms", True)
 
+        self.disable_msisdn_registration = (
+            config.get("disable_msisdn_registration", False)
+        )
+
     def default_config(self, generate_secrets=False, **kwargs):
         if generate_secrets:
             registration_shared_secret = 'registration_shared_secret: "%s"' % (
@@ -70,6 +74,11 @@ class RegistrationConfig(Config):
         #     - email
         #     - msisdn
 
+        # Explicitly disable asking for MSISDNs from the registration
+        # flow (overrides registrations_require_3pid if MSISDNs are set as required)
+        #
+        # disable_msisdn_registration = True
+
         # Mandate that users are only allowed to associate certain formats of
         # 3PIDs with accounts on this server.
         #
diff --git a/synapse/rest/client/v2_alpha/register.py b/synapse/rest/client/v2_alpha/register.py
index aec0c6b07..14025cd21 100644
--- a/synapse/rest/client/v2_alpha/register.py
+++ b/synapse/rest/client/v2_alpha/register.py
@@ -309,22 +309,16 @@ class RegisterRestServlet(RestServlet):
                 assigned_user_id=registered_user_id,
             )
 
-        # Only give msisdn flows if the x_show_msisdn flag is given:
-        # this is a hack to work around the fact that clients were shipped
-        # that use fallback registration if they see any flows that they don't
-        # recognise, which means we break registration for these clients if we
-        # advertise msisdn flows. Once usage of Riot iOS <=0.3.9 and Riot
-        # Android <=0.6.9 have fallen below an acceptable threshold, this
-        # parameter should go away and we should always advertise msisdn flows.
-        show_msisdn = False
-        if 'x_show_msisdn' in body and body['x_show_msisdn']:
-            show_msisdn = True
-
         # FIXME: need a better error than "no auth flow found" for scenarios
         # where we required 3PID for registration but the user didn't give one
         require_email = 'email' in self.hs.config.registrations_require_3pid
         require_msisdn = 'msisdn' in self.hs.config.registrations_require_3pid
 
+        show_msisdn = True
+        if self.hs.config.disable_msisdn_registration:
+            show_msisdn = False
+            require_msisdn = False
+
         flows = []
         if self.hs.config.enable_registration_captcha:
             # only support 3PIDless registration if no 3PIDs are required