I charge $140 for service call, which includes up to an hour worth of work.
I then charge $85/hr in 15min increments.

While I'm in the same boat as all of you in that I don't charge for phone calls, if I was to charge for said phone support, I think it would be an easy sell.

Informing the client that I'm only charging my hourly rate in 15min, or even 5 or 6 min increments.
That way a 12min conversation would be $17 (.2x85) charge, instead of $140 truck roll fee.