I suspect that when they offered you the new phone, they put you on a new contract, hence the increased rental. it seems that so far they are avoiding putting anything in writing and it may be that legally they are required to cancel a contract within a certain timeframe and are trying to avoid doing anything in the hope that you will let the timeframe lapse? Just a quess on my part, I may be completely wrong! I would definately contact them asap by phone and then follow up with a confirmation email to ask them to either put you on terms you are happy with regarding a new phone and rental, or to take back the phone asap. I would keep copies of all your emails sent to prove that you have tried to resolve the situation. I would also shop around for new deals and bear in mind that you never get anything for nothing! The brand new all singing and dancing phone that you may be offered will be covered in the cost of the rental agreement that you could be tied into for 12 to 24 months!
Cheers Sue!