Yes, it certainly left a lot unanswered.
Callout pay was introduced when they last rostered trainers onto a line month. At first, if they called you out for a training duty while you were off training, they had to pay the whole training allowence for the month.
That soon stopped, and they introduced a one-off payment per duty- I THINK it's basically your allowence divided by twelve.
If you were to get called out for twelve duties, you revert to normal training pay, including extra for any duty in exess of twelve.
Does seem to me that they will have a pool of "reserve trainers" and be motivated to put as many back on line as they can get away with.
When it starts is also a good question- I'm putting in a CRS bid next month just to CMA.