Tls & srtp

I have gotten the sangoma S700 to register with TLS and SRTP. Now I can make outgoing calls to via a SIP trunk and can call any asterisk function for example echo test, voicemail etc, And it uses SRTP as seen on the top right corner of the phone. Now when an incoming call comes in the phone will ring but as soon as I pick it up, it will Hangup the line and the call goes to voicemail. What could be causing that?