Hi looking for some help please.

Have had my iphone 3gs for 2 months now - has been working fine.  However just tonight, at home (reception is all good here) the reception bar said "No Serivce" However when i go into settings it finds both the vodafone and telecom networks, and there is a tick by the vodafone network option.  Yet it still says no service up the top left hand corner...

Rang vodafone, they refreshed and it worked again, then 5 minutes later - no service.  Have taken the sim out and it works in another older phone.  A telecom simcard works well in the iphone too, but my vodafone sim card doesnt work in my iphone...

I have turned it off and on (several times), have reset the networks under settings.  Reception comes in for a bit, but then it goes to "searching" then it says no service again.

Im stuck - and frustrated!.

Anyone had similar problems - any tips or advice please???

Cheers