If the issue is with just one 7942G phone and there are other 7942G phones which are working fine, then try deleting the phone completely from the call manager and readding it. Also, check by registering the phone with the publisher.
Yes, that's a good option. But try upgrading to firmware version 8-5-2 first instead of 8-5-3 directly as going to 8-5-3 might need a step upgrade. I have come across issues with customers trying to upgrade directly to firmware 8-5-2-SR1S , they need to be on 8-5-2 first, only then they can go to 8-5-2-SR1S, i think it applies to 8-5-3 as well.
I'm not able to access my old voice mail messages all of a sudden. The recording says something like 'the message is currently not available'. This has never happened before in all the years I have been using this system. I have t...