https://support.google.com/adwords/answer/2453991

What phone numbers can do
Encourage calls to your business by showing your phone number on your ad.
Display a clickable call button with your ad (on high-end mobile devices).
Cost the same as a headline click (a standard CPC).
For call-only campaigns, ads will only appear on devices capable of making calls.
How you benefit
Adding phone numbers to your ads makes it easy for people to call you on the go.
You can share these numbers across ads within an ad group or entire campaign.
Clicks on call-only ads exclusively drive calls to your business.
You can set numbers to show only when your business can take calls.
You can count calls as conversions. Learn more about call conversions.
Call extensions have typically increased clickthrough rate by 6-8%.
Example
You own a pizzeria in downtown Chicago, and add a call extension to your campaign promoting deep-dish pizzas. If a visitor to the windy city searches for “Chicago pizza” on her phone, and one of your ads is triggered, this visitor can click through to your website to read your menu, or touch a call button on your ad to get straight to your ordering line.

If you only wanted more phone calls to your pizzeria, you could create a call-only campaign. Your ads would only show on mobile devices that can make phone calls, and all clicks would result in hungry visitors dialing your number to order a slice.
Did you know…
Ad extensions are factored into how Ad Rank is calculated, so showing more relevant, better performing extensions can help improve your ad position. If two competing ads have the same bid and quality, then the ad with the more relevant extensions will most likely appear in a higher position than the other. Learn more about how ad extensions appear on mobile text ads.
Set up ads with phone numbers
Good to know
When you give preference to mobile phones, your call extension will be less likely to show on desktop and laptop computers.
If a campaign or ad group has only mobile-optimized call extensions, these may show on desktop and laptop computers. To avoid this, include standard call extensions of the same type as the mobile-optimized call extensions in your campaign.
Call extensions aren’t available for “Display Network only – Remarketing” campaigns, or “Display Network only” campaigns with the marketing objective of “Install your mobile app” or “Engage with your mobile app.”
Call-only campaigns are not yet available for campaigns targeting the Display Network.
Create a call-only campaign
If your goal is to increase phone calls to your business, you’ve come to the right place. Call-only campaigns allow you to focus on getting more people to click-to-call you straight from your ads. With these campaigns, you’ll be able to use CPC bidding based on the value of a call to your business. You’ll also be able to add your existing phone information to your new ads.