As far as SAT to IAH goes, you are prob just a bit too far to sit Short Call in San Antionio. The call out requirement for Reserves on Short Call is to be able to report in 2.5 hours. That being said, I am sure there are those that do (just prob not while on probation).

The latest routine is for the crew desk to basically give everyone (that doesn't get an actual trip) a Short Call assignment. Those vary in start times throughout the day and are predominantly timed to protect INTL departures that get delayed.

So, based on being over in SAT, being on Reserve would either be a QOL pain in the arse (drive up the road an hour while on short call) or a great deal of time gambling that you don't get a minimum time callout.

If you are actually within 2.5 hours, then the you are in a much less painful situation.

My advice when it comes to commuting to reserve (over 2.5 hours away), not just no, but HELL NO!