Apply logging from hotfixes branch to develop

Part of #4826.

Originally added in #11882.

synapse/rest/client/account.py

@@ -415,6 +415,7 @@ async def on_POST(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
             request, MsisdnRequestTokenBody
         )
         msisdn = phone_number_to_msisdn(body.country, body.phone_number)
+        logger.info("Request #%s to verify ownership of %s", body.send_attempt, msisdn)
 
         if not await check_3pid_allowed(self.hs, "msisdn", msisdn):
             raise SynapseError(
@@ -444,6 +445,7 @@ async def on_POST(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
                 await self.hs.get_clock().sleep(random.randint(1, 10) / 10)
                 return 200, {"sid": random_string(16)}
 
+            logger.info("MSISDN %s is already in use by %s", msisdn, existing_user_id)
             raise SynapseError(400, "MSISDN is already in use", Codes.THREEPID_IN_USE)
 
         if not self.hs.config.registration.account_threepid_delegate_msisdn:
@@ -468,6 +470,7 @@ async def on_POST(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
         threepid_send_requests.labels(type="msisdn", reason="add_threepid").observe(
             body.send_attempt
         )
+        logger.info("MSISDN %s: got response from identity server: %s", msisdn, ret)
 
         return 200, ret