Does it Cost money to use Yahoo Answers?

There is no contract(or even disclaimer) that indicates monetary obligation, so probably not. That doesn't mean that there wasn't for something else(just that it's not yahoo answers). Keep a records of all correspondence(save the email, record phone calls, etc.). Contact the company directly, but be sure not to accept any obligation(even verbally). You can send a letter that states that you don't recognize the debt and demand that they cease and desist. This will prompt legit companies to provide specific proof. If they've previously stated that contact implies any obligation, get a lawyer to do it. Entering into a contract, without knowledge of the contract, generally falls under non est factum.

This is personal advice, not legal advice. I cannot give legal advice. When in doubt, get a lawyer.

This sounds like a weak scam, but why risk anything on an assumption?

it costs money to get online, so yes

no cost other than data and totally fee if you are on wifi