Pay Per Call:
Bid as you would with Pay Per Click (but starting at a whopping £2 per call ($3.80)).
The adverts show a freephone number along with your details, there is no mention of your website as the idea is to get the person to call. Any URL will go to another page that has more info and a prominent 0800 number.
When anyone calls the number it costs the advertiser in the same way that Overture ads do, the simple 1p more than the person below you.
If the person calls back within 10 days there is no double billing (for Miva, other vary). The cost of the call is absorbed in the bid price but will probably terminate after 10 minutes or a set period.
That's easy enough, but..
There simply are not enough freephone numbers in the UK to cope with many publishers (as you have to have different phone numbers for tracking of each advertiser * how many publishers)
I see some exclusion on the way. If you don't own a very big property then you can probably forget earning through Pay Per Call.