Poor service / Wrong debit amount

Upgraded to a sim only contract recently. The plan was to go pay as you go but the operator convinced me to do a sim only contract with some airtime. The 1st debit was all good and it then increased. I did query this and it was corrected. Again the debit for this month is again much higher than agreed on (more than double). I tried calling but cannot get help, the last time I called today the operator ended the call without assisting. Now my request is, if they cannot keep to what I agreed with let us cancel this contract and upgrade me to a proper contract where I then get a handset and know what the debit will be monthly. I’m currently paying more than what a used to pay with a handset, it doesn’t make sense. Please help to get this resolved. I don’t mind to pay, but then I at least want value for my money.