This is a simple problem and Chip explained it well. If you are billing your clients by the hour (or fraction thereof) for your time, then bill them for your time. This is how most attorneys work. Pick up the phone and talk to your lawyer for ten minutes and you'll get billed for those 10 minutes.

If that seems like too much trouble to keep track of, then you probably have other "leakage" areas where you are not tracking, and thus not billing, for your time. To make it simpler for everyone (you and your clients), figure out what your average communication bill per month (or week) would be and start charging a flat administrative fee. Some months you'll "win" and others you'll come up short, but the customer knows they're covered no matter what.

Most clients are understanding when it comes to being compensated for your time. Those that aren't should probably be replaced.