How to solve no signal problem on htc 8525? - HTC TyTN II Smartphone

I'm using HTC TyTN II with Windows Mobile 6.1 as OS. My problem is my phone keep loosing network signal and occasionally when it get the signal, I can't make a phone call. The signal is there, but when I make a phone call, it will disconnected at once.

I have literally just solved this issue oin my tytn II, i am on the 3 network, i have had the problem all day, not through updating software etc, it just occoured, checked sim in another phone, all ok, checked settings in tytn, all ok, removed sim and restarted phone, laft it on (without sim inserted) for about 3 mins, then turned off and re-inserted the sim, all is back to normal now.