Don't measure the same levels of activity. An internal person might legitimately produce dozens of quality calls a day, and an outside person only two or three. Consider different incentive plans. You might want to reward inside people for number of pieces of business and average size of business. However, it might make sense to reward outside people based on type of business, improvement over prior year, new markets penetrated, and so on, based on the company's strategy.
Outside people have more potential to influence the overall business thrust than do internal people.
Finally, never mindlessly transfer people between internal and external sales. They are discrete and different pursuits, requiring separate skills. Few people possess both, and I've seen too many excellent people in one area ruined when transferred to the other. Consequently, do not build a system wherein an internal person must transfer to outside sales if he or she is to advance or make considerably more money. You'll ruin two jobs as well as your own well-being.