problem with k800i

there is a problem with my k800i, it sometimes can't recieve or make a call, if some one makes a call to my phone it will say "this number is not avialible" and it not because I'm on the phone or it's switched off. but when someone try it 3-4 time it would connect. samething when I try to make a call it wouldn't connect until i do it 3-4 times and sometimes it work the first time. all the other functions work, and I have tried different sims but sameproblem.
so does anyone know what is wrong with the phone and how to fix it.

thaks in advance