When I worked for a large company, I was in charge of analyzing promotion campaigns. It was relatively easy to see how sales would increase or decrease depending on a particular promotion. You had to consider other factors such as weather, year over year comparable sales, and economic indicators but one can see a direct impact on sales depending on the promotion.
You can do a similar analysis where you increase or decrease the adword campaign and track your call activity. Weigh this activity against business drivers for your business as well as compare it to the same prior period. You may be surprised by the results and this may help you decide whether to continue on a particular promotion or try something different.