The first time the extension gets registered, but soon it gets offline (grey state) when you check on the monitor page.
Or the IP phone shows registered, but the PBX monitor shows it is offline.
1. Unstable UDP connection on the client-side.
This cause makes the client SIP device could not the SIP Keep-alive packets from PBX. PBX then marks the client SIP device as "offline".
1) Ensure the client-side has stable Internet access.
2) Enable the Keep-Alive function for the SIP account on the IP phone. Configure the Keep-Alive interval with a proper value like 30 seconds or less but not less than 15.
A reference of Snom IP phone:
3) Reduce the Qualify Frequency value on PBX to a proper value like 30 seconds or less (but not less than 15) till the issue doesn't happen again. If this option doesn't help, try to revert it back to the default value. This option is not that recommended because it will consume more PBX performance.
For S-Series Cloud:
For P-Series Cloud:
4) Configure the UDP connection timeout of the router or firewall on the client-side for more than 60 seconds. Because PBX sends the SIP Keep-Alive packet every 60 seconds (a little less than 60 seconds, it could be 50 seconds or less) by default. A reference for SonicWall.
5) Change the extension transport to TLS or TCP.
Note that remember to change the transport both on PBX and IP phone.
2. Incorrect Firewall Settings on the client-side.
The firewall may have some packet control for SIP communication.
Please seek help from your IT to allow the SIP communication to our Cloud PBX.