[jmspool]

If it's a retainer, then they pay you up front for a certain number of hours you'll be available. This can be at a higher fee than the other work.

Another alternative is to just have an on-demand hourly rate that is higher than your normal rate. You could have pre-scheduled "office hours" at the regular rate, but if they want you at other times, they get to decide to pay you a little more.