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.
If official support is required please check how to phone or open a case here
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