Start by measuring how many calls per hour to gauge what your people are doing now. Then you can introduce a pay plan that drives toward a little more than that, and a little more, and a little more, and....
I made that switch after Jerry Thibeau (Phone Up Ninjas) suggested that 8 per hour was ideal. We got a few folks all the way up to 12 at one point, but couldn't maintain that pace. Our laggards averaged around 6 to put things in perspective. The base pay was done on the average number of calls in/out per hour with bonuses based on number of appointments that showed. Internet leads helped to raise the number of calls per hour, so we didn't have to make that a stand-alone metric in the pay plan.
P.S. I still have to thank Jerry Thibeau all the way back to (I think) 2006 when he made this suggestion. It was probably the most significant performance increaser I ever implemented in the BDC.