Blacklisted mobile device

After the end of my contract i upgraded and got another device, i then realised that the old device stopped getting network reception on any sim card i inserted. I called the call center and was informed that they have accidently took my devices IMEI number and put it on someone elses profile who has a bad credit record. Now i cant use my phone that i have paid for, for 24month and when i spoke to agents they want all sorts of documents and i feel that isnt fair cause this isnt my fault