I'm not a technical guru, but these may be some options:
1. call your provider and request a new number. You don't want someone else's number...as you well know from your experience
2. go to a phone store and ask them to change the SIM card (don't ask me what it is, but I think it's the microprocessor that has all the phone properties on it). The person at the phone store should know what you're talking about. I don't know what type, model, or make of phone you have, but if you go to (or call before you go) a phone store, they may well be able to help you.
3.Just because the account name and address changed, (I'm assuming that's what happened) that doesn't necessarily mean the system knows that the number is no longer associated with the original owner.
4. If you have Caller ID for in-coming calls, you answer only the calls you want to answer. Just because the phone rings, doesn't mean you must answer it. :lol:
There's no reason should be getting phone calls for someone else on what is now your phone.