Problem with VOIP

Hello all,

I have problem with FortiVoice FVC-40D2 (firmware version: v5.2.5,build95). So my Voip is working 2 mins and 2 mins is not working. Checked the logs and it says sendRegistrationNotifToCallp:1032 (phonenumber , SIP_RESOURCE_NOT_REGISTERED) when this is in logs the voip is not working but when is SIP_RESOURCE_REGISTERED then is working. I googled but cant find the solution.