Political ads with phone numbers

[Mark Laurence <mlaurence@mindspring.com>]

A lot of political ads this year include a phone number, and I'm 
wondering why.  Until this year, it seemed most ads were content to 
pump up the candidate or blast the opponent.  This year, many of the 
ads include a quick message to "call Mitt Romney and tell him you don't 
want him to throw old people out in the street" or "call Shannon 
O'Brien and tell her to stop throwing away the pension money."  
Obviously they're really attack ads, but the phone number seems almost 
like a disclaimer.  They don't repeat the phone number or truly 
emphasize that part of the message.

Is there some law that's making ads get worded this way?  Most, maybe 
all of them, are paid for by organizations and not the candidates or 
their own committees.  Why the sudden interest in getting people to 
call the candidates?

Mark Laurence