This would be a great feature for my agency as well. I have quite a few clients who pay per lead and having the ability to charge them a fee per call would be a huge timesaver.
CallRail has the ability to tag calls automatically using keyword spotting and conversation intelligence. Setting some simple parameters (ie. length of call, keyword tags) would give the ability to charge the client appropriately for the call based on the service offering. For example, residential garage door service leads could be billed a one rate and commercial leads billed at another. Or a broken spring repair would be one rate whereas a garage door opener installation could be another.
I spend too way much time and money invoicing clients (myself and that of a VA). Ability to grow agencies based on pay per call would be so much easier with this automation in place. Another idea would be the ability to bill clients based on the number or calls (every 10 or 20), or based on an interval (7, 14, or 30 days).