Our group compensates for the call. We used to compensate by case, but the utilization is so unpredictable and no one likes sitting at home all night waiting for the pager to go off and not getting compensated just because it was a slow night. Day coverage gets minimal compensation (you're already at work), night coverage gets more, and weekends (Fri PM to Mon AM) are done in one larger clump. This is not part of the quoted base salary, but is divested monthly based on the number and timing of the calls.
With a smaller group with frequent call, you could just compensate by case, and expect that over a fiscal year the variances will average out. Or you could could pay net proceeds out each month as "bonuses" split three ways. Depends on seniority issues, and whether your calls are split day/night or something.
