Still want money for a ported no

A lady from Cellc just phoned me asking when I am going to pay the outstanding R1900 on my account. I noted to her that the number in question was cancelled and ported to Vodacom on the 10th of November 2015. They are asking of me to pay for something that does not exist... She also noted that the number was ported out of Cellc so what now?!?!?! She then told me to phone vodacom as they did the port, but how does that have anything to do with Cellc's accounting? I am however not liable to pay for services not delivered. I need this sorted as soon as possible.