Just make sure your yell listing has A) a unique phone number which is usually provided by yell and B) your web link should go to a specific squeeze page or landing page which can the be easily analised. simple to trac your online data
i got a free listing on yell.com worth £2000 and got about 5 calls off it. the rest of the calls were cold calling sales calls.
in fairness to yell though, we are situated inbetween areas so it doesn't quite work for us.
ps... be careful they don't try and forge your signature and sign you up for £2000 worth of advertising. they may do this and then they will send you lots of final reminder letters while you're arguing that you didn't sign anything. low lifes.
personally, i wouldn't urinate on them if they were on fire but i am a bit biased against them.