4s showing no service on SOME networks

Hi. Funny one here.

I have a 4s which i think is locked to vodaphone uk (according to my mate) and sure enough, on updating to 7.0.3 it activated with a voda sim card. However there are no bars showing although the 3g symbol is there. Checking the mobile carriers, voda is listed. This happens with 2 voda sim cards which work in other devices. The odd thing is that ive tried sim cards from other netwrks which show good signal, although it wont activate with these cards (gives invalid sim error).

So the thing is, the sim cards work in other devices, the phone shows signal with other network cards, albeit wont activate as the phone is locked to voda, but these cards wont work in this phone.

Any ideas or thoughts?

Al