*****ulent device contract

I had contracts with Cell C. Just when I was celebrating my contract-free life, they opened a new contract for a phone or device valued at R3500 a month, and they claimed that I was the one who signed up for this contract which I did not. Now I called them yesterday about this issue and they said I have a contract with them (like who signs up for that much just for a device). The tricky part is they said I must not freeze their account they will investigate. Which means I'll wait and look at them taking my money while they do the so-called investigations.

Any suggestions as to how to deal with this matter?