I've legitimately lost a credit card and had my bank call me about fraudulent transactions. I didn't know the card was missing until they called.
Instead lookup the fraud/accounts contact number for the bank and call them yourself. - aka start the interaction with known good information
The telco's themselves should be held legally responsible for the fraud in these cases.
If I hang up a phone, I expect that to be the end of it, period. Yes, that includes landlines.
Unless you know exactly what you're doing, what you should do in the case of any over-the-phone problem like that, is first contact someone you trust and knows what they're doing. Your own bank, an expert of your choosing (and not chosen by the stranger on the phone), or a knowledgeable friend or family member.
Never trust the stranger on the phone.