Call out on ready reserve is to return the call within 15 min; then be at the gate within 15 after that.

The only time when per diem is earned on ready reserve is if you are put onto a pairing (that begins calculation of your time away from base) during that duty period.

You can also be escalated to ready reserve while sitting on home reserve if Crew Tracking has "exhausted" all airport ready reserves for that shift. When you are escalated, they cannot make you sit past whatever shift the previous reserve was originally sitting. There is no further compensation for being escalated.

After reserve lines are published, reservists submit a reserve type request form indicating what type of reserve they prefer. Crew planning will generally honor this request based on seniority and scheduling effeciency.