Faulty billing still not sorted more than a month later!

A contract was upgraded as well as a new contract taken out - ONLINE. Charges were wrongfully billed. Have been trying to sort out this issue since 05 September. First we couldn't get an answer. Then eventually had a very helpful lady phone and we sorted it out. The matter still wasn't resolved. Another lady phoned and it sounded like the matter was going to be sorted. STILL this matter is not resolved more than a month later. This is becoming an absolute ridiculous situation to handle and to try and sort. Fees needs to be reversed. As simple as that. What do we need to get this sorted?! A lawyers letter?! I am trying to assist my elderly parents who live almost ****m away. My dad can not speak English fluently. Can somebody just please sort this matter our for once and for all.