Okay, so what I'm thinking is that you would have to ask your phone company about how to obtain a few phone numbers for the one line. Either that or set up a few phones with a number each. I think on really busy days (such as weekends) you would probably need some help (another person) in fielding these calls. You don't want to keep people waiting.
Also, you would need to get as much information as possible from the people placing the ads, so that when people ring up with questions you can answer them all.
I think it is actually a clever idea, but it would need to be set up quite precisely. Also, what would you be charging for your services? Some people may not like having to pay for a classified ad in the local paper and THEN pay someone to field calls for the ad.
Just a few things to think about. :-)