If any one has arrears with TSTT they are usually very flexible with re-payment terms. A neighbour of mine was away for a while and his kids ran up a really high bill and they put him one way.
He went into the office and was able to negotiate payment terms which were quite reasonable and allowed him to pay it off over time.
Also, I had overpaid on my landline bill and was able to transfer the extra credit onto my mom's phone (which was also on my name).
However, I dont think that you could ask this of a CSR you may need to talk to a supervisor or a manager. Please dont expect to call a CSR over the phone and expect them to negotiate terms of re-payment with you. Remember a call centre usually deals with routine customer issues. If you have to negotiate then I would suggest that you go in to the office and talk to a supervisor or manager.