A quick google search turned this up:
Phone Numbers in Text Ads - Please Help - AdWords Guidelines | Google Groups
Surprisingly, second response down is from Google, and YES you can put telephone numbers in google ads. However, people are reporting on the thread that it isn't really effective.
Look at it this way. I'm surfing the net for a company, am I going to find a pen and paper, write down the telephone number, go to the telephone and ring them? Or am I just going to click the ad? I'm probably going to do the latter.
However, some sectors it could work. Do I agree with it? Of course not, it's be jepordising my adsense income. But there's really not a great deal I can do about it.