It sounds like you have a session refresh problem with the provider.
2 ways to fix this:
increase the timer extremely high
find out what part of the refresh is failing.
15 minutes points towards failure because 1800 = 30 minutes, and the refresh happens at half the session. You probably have a CUBE bug you're hitting in regards to re-invites, or do not have 'midcall-signalling passthru' configured, or something similar.
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...