We have two different recent threads asking if you are allowed to place your phone number in your Google ads. The two threads are at Google Groups and WebmasterWorld and in both threads, a Google representative came in to say, yes it is allowed.
For example, a search on new york dentist has an ad with a phone number in it:

Google said, "Yes, you may use telephone numbers in AdWords text ads. It is not against AdWords policy."
But does Google recommend it? It depends, for some advertisers it works well, Google said. But for others it doesn't work well - people still want to "click and learn," Google said. Google recommends doing a simple A/B test, "with two ads in the same ad group - one having the phone number, and the other without."
Testing to see what works best for you is a great idea.
Forum discussion at Google Groups and WebmasterWorld.

Comments:
Grant Glendinning
12/07/2007 08:07 pm
I'm not sure if that is entirely true for Google.co.uk - we did a 30-day 'tester' on Adsense and put our phone number in the description, "Garden Furniture at Discount Prices - Call today 12345678910" - we subsequently got our ad campaign removed for a 'breach of terms'? Hmm, perhaps this is only a Google UK term, as I cannot see any other obvious reasons the campaign got removed by Adwords.
John
09/27/2009 07:58 pm
I'm a website owner and i block ads with phone numbers as they potentially don't bring me any click revenue.
senguttuvan
11/14/2009 05:35 am
my number ; 9629958588
le juge
05/18/2010 09:27 pm
I am running several campaigns with phone number included in description and never had any problem
johnjones
07/02/2010 02:00 pm
I run phone # all the time if people want to call the # is right there. Plus it saves you clicks. Who cares about maggots wanting traffic to their site, tuff cookies maggot boy
languages4free
02/06/2011 02:20 am
i block all ads that have phone numbers in them
Steve
04/27/2011 12:33 pm
Google puts a phone number in its own ads. Here's an example: $75 Free In Advertising Gain New Customers In Just 15 Mins Call 1800 988 571 to Sign Up Now! www.Google.com/AdWords