The way I look at it is, you're on-call for your client you need to be compensated for that - usually a flat rate or reduced per hour rate. This is charged whether or not you answer any calls. The charge a per call rate based on the average length of the calls based on the others suggestion or you could charge a rate for a call that is under 5 minutes, another rate for a call 5 to 10 minutes, etc....
Or simply charge your regular rate when you take a call and bill accordingly, but definitely include an "on-call" rate, you're there for the client and doing a service for the client.
This is a service I don't offer my clients just for this reason and when I do have clients request this service I usually refer them to
CallRuby which is an answering service that works as receptionists, their pricing is reasonable.