I've a feeling it might be blacklisted in that case

Which means you'll need to try and get hold of the network providers themselves to see if:
a. it is blacklisted
b. if it's possible to lift the ban on the IMEI of the phone.
If it's possible to try someone else's sim card in there too, that will probably help confirm whether it's blacklisted or not if they get the same problem.
You can call Orange 0800317720, or Vodafone on 08700776655 - but I'm not sure what the T-Mobile one is I'm afraid