MTN upgrade *****

Mtn upgrade over the phone is dangerous. One of their agents called me to upgrade my lines and promised to keep my lines as Top up. Now end of the month , I get a surprise bill of R1800. Their call center agents are not willing to help. Every time I call they tell me they transferring me and just cut the call. They become impatient and rude. I've sent emails and although they say their turn around time is 24hour, they do not honor that peomise. Im a pensioner and they *****ing me into paying for something I did not agree to and something I know nothing about. I even reached out to their ***** dept to try get help, but no response from MTN, they just want money and dont care about frustrating a loyal customer. I've had these contracts from 2012 and never had a problem until they decided they want to boost their sales with my pension.