02 XDA Mini S - call issues

I'm in England and I've just inherited an 02 XDA Mini S mobile phone,
but I'm having problems making calls on it. I have tried using it
with an 02 pay as you go sim card, but the device claims that it fails
to find a network. If I switch to using a vodaphone pre-pay sim card
then it finds a network just fine, but if I try to make any calls it
fails to ring the destination number and the call is reported as
ending on the XDA with a call time of 0 seconds. The odd thing is
that it receives texts though.

I've tried a soft reset and a hard reset, but the symptoms persist.
Can anyone give me any clues as to what the problem might be and how
to fix it.

Thanks.