Create a billable unit (some places use 6 minutes, some 10, some 15) - make sure that these clients are aware that a phone conversation or small task will be charged at the billable rate. Keep a record of each task/conversation and add up the units and hand over a monthly invoice.

Yes, this means that three two minute tasks for three different customers will mean you're charging 18 minutes (for 6 minutes of actual work) - but this is pretty normal.

The important thing is to do this from the outset - that way, you're not disappointing anyone by suddenly charging them. Also, it reduces the time you spend on, shall we say, simpler queries.


Minor revision to clarify:

If, at the end of the month you only have a couple of units work against a client, you should probably waive it. If you get several hours (that's only two or three 15 minute sessions a week), then you want to charge them those several hours.

Of course, if they're paying you for a bigger project (or just paid for one) you might choose to waive a few extra hours.

Overall, though, if you are up front about it, then choose to waive it, then you're going to get brownie points. If you charge, you get your time reimbursed. Win, win.