The 15 minute call back window does not add 15 minutes to you 3 hours. It's 3 hours to push from the initial attempt to contact.

If you are on reserve, what difference does it make if you get the extra 4.5 hours? You are working for 72 hours until you go over. Have you ever broken guarantee on reserve?

Not to point out the obvious, but most airlines do not have trip averaging. Most airlines, worst regional up to double breasted Delta, have block-or-better by leg...not trip averaging.