Well, there is a hard and fast rule in the contracts about this . . .
In reality, the 30-minute call is the customary call. We already have accepted examples of calling an actor in earlier (for a fight call) or later (some SMs will call an Act 2 actor at Curtain for example). There are rules about adding rehearsal hours prior to the half-hour call.
In the case of the LORT contract, I have had it handled in different ways, depending on the reason, the rep, and how it was presented to the actor . . .
1) As long as the breaks prior to the show call, and the time worked didn't extend passed the 3.5 hours for the show - we were fine.
2) I have the time taken from the available rehearsal hours that week. So if an actor was called 15 minutes early for 8 shows, which adds up to 2 hours, I was able to rehearse that actor 8 hours instead of ten (or 18 instead of 20 if they were continuing on to another show).
3) It was paid as overtime.
The real answer for a stage manager is if they think an actor is going to be called prior to half-hour, they need to discuss it with general management, and eventually figure out on their contract how the union would address it.
But, an actor should have to work longer because, for an example, a producer doesn't have enough people to put on wigs.