Make a Google Voice account. It's totally free and you get a free online phone number that actually works. You can text and call people through it for free as long as you have internet access. If you already have a Gmail or Google+ account, it's easier to just link your Google Voice account to those. You can then use the phone number you get from Google Voice as a phone number for your Yahoo Answers. That's what I did when I signed up for Yahoo Answers, Facebook, and all that; I simply used my Google Voice phone number since I didn't want those sites to know my actual cell phone number.
If Yahoo are going to call you to verify your account, a fake number wouldn't really be helpful. If they don't use the number to actually contact you, but simply as a check to verify whether you have a genuine phone number through an algorithm, you could can get a makeshift phone number online...
How to get a phone number online in order to sign up for YahooYou can get a free number through Skype http://www.skype.com/en/features/online-number/
Ar as the person above said, through Google voice, though this is currently only available if you are in the US - and you do need to register your real number with them in order to use the service.
Another simpler alternative is to provide them with an old phone number, perhaps one registered to a device that you don't use anymore.
I have to admit that most online sites that demand a number, don't actually get my real number, and that only becomes a problem if they need to contact me for some reason. However, I can just tell them at that point that I have changed my number and forgot to give it to them when it changed.
Personally I have never had a problem with using an old number for things like that.