A very good question and thanks for bringing it to us.
You indeed have panel data and it is likely that PROC PANEL contains some estimators that will help you find the impact of the campaign (both calls and coupon size). Since it appears you only have information on customers using coupons, you will likely only be able to estimate the impact of changing the coupon size. But that should be ok.
Now for the hard part. If you are willing to assume away any autoregressive process and are willing to assume the impact of the program is say, 3 periods, then create three lagged values of coupon, include those as regressors and estimate a two-way fixed effect model. The general syntax could be found here. SAS/ETS(R) 13.2 User's Guide
However, if you are unwilling to make that assumption (no serial correlation), then you have to move to a more complicated estimator to get consistent estimates. An example of that can be found here. SAS/ETS(R) 13.2 User's Guide
As a first, pass I would try my first recommendation. Best of luck.