A quick way of checking if the phone is still registered to the SIP Provider is looking at the Line Icon:

More detailed Information can be obtained via:
Press the Menu Key => select Status => select Lines
this menu will offer you now either the Line Information or the Server Status

Both of the above should provide you with a countdown Information showing the registration/subscription to the SIP Server.
If above shows nothing or no registration Information the Phone has lost it's connectivity to the SIP Server. Please use this post => here <= to attempt and troubleshoot this.
----------------
The title Polycom Employee & Community Manager is a community setting and does not reflect my role. I am just a simple volunteer in the community like everybody else. My official "day" Job is 3rd Level support at Poly but I am unable to provide official support via the community.
----------------
Notice: This community forum is not an official Poly support resource, thus responses from Poly employees, partners, and customers alike are best-effort in attempts to share learned knowledge. If you need immediate and/or official assistance please open a service ticket through your proper support channels.
Please also ensure you always check the VoIP , Video Endpoint , Skype for Business , PSTN or RPM FAQ's