There is a new bid adjustment that Google is adding to AdWords specifically for calls.
With this bid adjustment for calls, it informs how often call information appears in mobile search ads
From the announcement:
For example, if you’re a travel advertiser, you may see higher order values from calls because it can be easier to cross-sell rental cars, group tours and other vacation add-ons during a live conversation. Raise your call bid adjustments to show call extensions more frequently and drive more of these high-value call conversions.
The bid adjustment range is -90 percent to +900 percent.
Keep in mind that this option is available in in any campaigns accessed (not created in) from the new AdWords interface. Calls are also listed under “interactions” in the UI.
Bid adjustments for call extensions is just the latest feature to roll out exclusively in the new AdWords experience.