Calls maybe cut short after 30 seconds because some routers are set up to detect attacks. Unfortunately the nature of VoIP simulates this behaviour and routers detect this erroneously.
Please log a support ticket with the following information:
1. What make and model of router are you using?
2. Whether there are settings in the router you are using which refer to any of the following:
- Intruder detection
- SIP ALG
(These settings are located usually under the WAN or Firewall settings)