Selling my mobile phone tariff

[Barquentine]

I want to sell my Personal Call mobile phone contract originally from 'One-to-One' now known as T-Mobile. It has free landline calls (nationwide) for 108 hours per week (7pm-7am Monday to Friday & all weekend). However, T-Mobile tell me that, although I was once allowed to sell that, they have changed the rules and I can no longer sell this particular tariff. Does anyone know if they can change the contract terms unilaterally like that, and if so, can they really stop me selling my tariff - which I thought was transferable?

[Postdog]

Can't answer, and to be honest I have never heard of selling such a contract but

a) your original contract was with one to one, and they no longer exist, so you have (to me) basically accepted any new contract with the new supplier rather than declining or terminating then

and

b) what happens if you sell it and they don't honour it - you are out of pocket having to pay them back.