I can confirm disabling the sip module on sfos 16 xg's in our environment resolved intermittent sip registration problems for yealink & aastra phones we use with a voip service provider.
We are currently have issue on sip registration on just one yealink (model t41p) but for other yealink models else are fine. Does it fixed transfer calls or choppy/static calls?
if it's just a single phone not registering and isnt resolved by unloading the xg sip module + power cycling, test that phone from a known good switch port that's working for another phone to isolate further and if it still fails there it's time to contact the voip service provider. choppy/static calls are generally a lan or wan network problem, high latency along the path from phone to voip provider, or saturation of the available isp bandwidth. congestion on the lan could also be responsible with enough going on internally. icmp response monitoring of the lan & isp side of the connection will help isolate the culprit. end to end latency needs to remain well below 150ms. i havent setup netflow or snmp in our xg's yet since we're still in early stages but from experience with other vendors looking at interface utilization and sending netflow to a decent traffic analyzer will give better visibility into what traffic is responsible for high latency or saturation enabling you to act on it. assuming your voip service provider is across the internet and not a system you control internally, to overcome minimal isp upstream bandwidth check into qos & prioritizing outbound voip as it hits the switches & xg using the yealnk dscp tagging or ip's. inbound voip traffic quality control is trickier but limiting other unwanted user traffic factors into the result. if the isp connection is just garbage even with no traffic passing over it which should be revealed through icmp response monitoring of wan ip with lan disconnected, then that would be the first problem to address since everything else depends on it.
