Getting a handle on your call coverage means you must benchmark against your local and regional competitors, and if you're a large system, you should do a national benchmark too. Benchmarking takes time, which is why it isn't done nearly as frequently as it should be, so if you are scratching your head over when you last looked at your call compensation, it's been too long and it's time to do it again.
There are numerous compensation surveys to turn to. The rule of thumb is that you should use a minimum of two, but more may be better. Start by looking at the large industry compensation surveys, such as AMGA (which has a call coverage survey as well as a larger compensation survey), MGMA, and Sullivan Cotter. These three, well-known benchmark surveys are useful tools, however, keep in mind the data is compiled approximately a year before publication, so the numbers may be a little stale.
Once you cull a call coverage average from these surveys, review a couple of pulse-point surveys. This data is compiled and released quickly (usually within six months of the surveys) and can help gauge the current compensation scene.