Mobile not making calls

[stevie21]

I've just bought a backup mobile which I need to use for 2 or 3 weeks. I can send & receive texts OK on it but it won't make or receive calls. As it's 2nd hand, someone suggested that it may actually be lost/stolen and the owner has had the handset barred. Is this likely (I'd have thought that texts would also be blocked) and what exactly do I do now? I'm leaving the country for 2 weeks on Sun night so going back to the shop isn't as easy as it would normally be. If it helps at all the phone is a Siemens A55.