Android SIP client registration


I am using Android 5.0 and Asterisk 12.3.0.

I have two users in sip.conf





If I set up my android SIP client with transport type = tcp, I can see it register on the asterisk console and all works fine.
However if I set up the SIP client with transport type = udp, I can see no registration attempts in the asterisk console, and after a while the client times out and says registration failed.

Any ideas why this is so would be much appreciated.