Does anyone have any experience or ideas about paying commission for leads generated by telephone?

My client runs Smile 60 - Laser teeth whitening in 60 minutes which is a laser teeth whitening company and this generates leads via the telephone.

I am thinking that if an affiliates comes to the website via a certain code in the url string a different telephone number could be displayed so being able to track the leads.

Any thoughts would be appreciated.

Regards

Jason