Ex T-Mobile Nokia 6233 won't work unlocked

[Gordon A]

A mate has a Nokia 6233 that was on T-Mobile but got it unlocked. The thing is, though, when another SIM is put in it, the 'phone recognises it and the network, but won't do anything. Attempting to ring out gives and error and trying to ring into the 'phone gives an error, too: something along the lines of "There is a fault." When SIM is put back into another 'phone it works fine so there's obviously something gone amiss with the 6233. Anyone any ideas? I don't know what the 'unlocking' involved; he just said he'd had it done!!