The iphone 3gs can use both micro sims and older size ones. Networks encourage you get a micro one as its a standard fault find process with tech suport if you use an older one.
Put you sim card in, edit the cell data settings and see if you can access the net. If you can but cant make calls, then it most likely black listed. iYou can still use data services with a blacklisted phone.
If it was blacked listed there are ways around it using a orange sim card
