Tell teachers with whom you work, too. Make sure they think to request you if they must take extended leave. It wouldn't hurt to give them a business card, if you have one. Call school secretaries when you will be out of town or on vacation. Make sure they know when you will return, and call them as soon as you do.
Chances are, at that hour, it's a subbing call.
Most likely, the placement person will assume you are already on another subbing job and will not hold it against you. Keep a file on each school in the district. If you get the opportunity, copy class lists, too. Chances are you will sub for the same teachers again. Get to know each school's rules and expectations as soon as possible .