Be prepared to be called at any time of day or night! It is reasonable to expect calls from your agencies anytime from 6:45am asking you to work that day.
A call to work that day may come as late as 11:30am too, as teachers (slaves to the job that they are!) will often turn up for work when they really should be at home with Ibuprofen and TLC!
Times in the evenings that you will be called largely depends on your agency. Some would rather call you at 10pm, following a conversation with a supply co-ordinator and give you a few hours notice about the work the next day than wait until the morning.
Calling Them
An awkward one to begin with. Do you call your agency and let them know you are free today, Friday, even though on Monday morning you called them to let them know you were free all week?
Yes! Especially if they know you work through more than one agency, they will not assume status quo after a few days.
If you are unavailable for supply, do let the agency know. If you're already booked, and cannot make it, let them know as early as possible in the morning, so they can sort out someone else to teach.