Google sent out an email to most of their advertisers a few days ago, explaining that they are launching click-to-call on mobile devices with full HTML browsers (e.g. iPhone, Android, Palm WebOS). It appears that this will be started automatically and as soon as Google launches this, I and other advertisers will have their phone numbers show up on mobile devices and be charged a cost-per-call when used.
In fact, the cost-per-call will be the same price as you would pay for a cost-per-click.
What if you do not want to participate? Easy, "remove the phone number from the business listings included in your campaigns targeting mobile devices," said Google.
Here is a picture of the email Google sent their advertisers:
Forum discussion at WebmasterWorld.