If you want to include a phone number in your AdWords ad, you'll need to use the call extensions feature. Starting in April, Google will begin disapproving any ads that include telephone numbers in the ad text, according to a recent policy update.
The official word from Google:
In the next few weeks, we will no longer allow phone numbers to be used in the ad text of new ads. Advertisers who would still like to promote phone numbers in their AdWords advertising can use the call extensions feature. We're posting this alert now to provide adequate lead time to make ad changes.
In April 2013, we will begin to disapprove ads that were using phone numbers in their ad text before the March 2013 policy change.
While an ad like this is currently showing, once April rolls around that ad will no longer be allowed in AdWords:
Advertisers will continue to be charged for a standard ad click every time a mobile user clicks on the forwarding phone number link or "Call" button (this is not a new charge), both of which can be seen in this screenshot:
One cost Google is eliminating with AdWords enhanced campaigns upgraded call extensions is the $1 charge for using a Google forwarding number on ads that appear on desktop searches, such as this one.
Using call extensions with a Google forwarding number allows you to measure conversions. You can find details such as phone impressions, phone calls, phone-through rate, and average cost per phone call in the Campaign and Ad Group tabs.
According to Google, "your ad group will have to receive a minimum number of clicks to be able to show call extensions, and a minimum number of calls to show a Google forwarding number."
Google provides more information on setting up call extensions.