Getting 503 Service Unavailable on 3g

I am getting 503 error when trying to call.I am using pjsip as my client on phone.
I am using Asterisk 11.5 and TLS signaling with SRTP media.My certificate is untrusted and I have switched on setting to ignore untrusted certificates.

Sometimes the call goes through after registering once but then starts giving 503 error.
I tried using the tcp dump on server and it shows that RST packet is being sent back from server.
Everything works fine when I use Wi-fi.