Firstly I would advise that you start shouting very loudly at your service provider, especially if your calls are dropping after 30 seconds!!
Secondly, how are the phones connected to your VOIP service provider? Are you running local NTP? When you say you have changed the offset via primary setup...is this in the config files?
What PBX and phone software are you running?
There are alot of factors that may be in play in your current setup so understanding your layout would make it easier to rule out certain things..... Are your phones on DHCP? Do you have a local switch/router? Local NTP? PBX - version and location? Phone software version?
The time I would think will be fairly easy to sort out its usually just an NTP/DHCP thing, the calls dropping after 30 seconds is another thing and may require a more in depth analysis of your VOIP/LAN/WAN setup....