Billed for system error that created duplicate acc

Took out cellphone contract on 2-1-15. System error prevented sim card being activated. Another sim card opened & activated. Assured that 1st card did NOT activate. Agent saw next day that 1st card HAD activated & logged case to cancel card. I was not told. End Feb inv was very high so arranged to pay an amt of R400 by EFT to avoid high month end debit. Was smsed acc details and ref no. Assured that not necessary to send proof of payment as ref no. was sufficient. I since discovered that a completely incorrect ref. no. given. Billed full amt of R807.59 end Feb. Rec'd March invoice shows R4380 termination penalty for sim that had been activated in error. Noticed that I'd been billed for 2 contracts in Feb (thought it was admin fees that made acc so high). Called & was given ref no. & told I would get feedback in 72 hrs. Did not happen so went into the branch where I took the contract. Agent very helpful & called acc dept who will e-mail acc manager on Mon 9th & copy me in. Told that they \hope"this can be reversed before billing on 31 Mar. So I may have to pay penalty for YOUR system error & be re-imbursed later. I DO NOT have R4380 extra in my acc. I am beside myself with worry. """