Very different from a briefcase wanker or buswanker but remarkably bawbagish.
"...Who are you and why are you calling me?"
"eeehhrmm.... who ur you??"
"Not telling you!"
"well yer a wanker then!"
Both parties are aware who the real wanker is, but only one is enough of a wanker to actually state that the person receiving the call is a wanker.
Said wanker - the phone book wanker - will then get bored and either do what wankers do best or go back to his (probably sticky) phonebook to find another hapless victim.