I think your'e on dodgy ground
if the contract (and the fact YOU sent FOR the item means it is a contract) states return to xxxxx within whatever
and you didn't - you are in breach of that contract.
the old sent you a letter - forgot to include the cheque ploy is as old as the hills - so even when it's genuine - it's a job to prove.
the fact that you DID send it back to them (even if it was the wrong address) should count for something - but I suspect in law it counts for nothing and you are at the mercy of their good will.
it seems petty - but the mobile companies are dealing with huge numbers (of young people) and a fair proportion will "know the tricks" I imagine they will have a pretty inflexible attitude