To answer your question :- "are you paid if you are called in?"
Yes you are, however that is not the point. On a contract for Ryanair you are paid per block hour you work and that is it, nothing else, nada, zilch.
If you are sick, on holiday or not used on your standby then no money.
More to the point, the place you have to be to do your standby is very unlikely to be anywhere near where you live (probably another country) therefore you are incurring hotac costs etc, for no return.
It's very harsh but then again nothing is hidden when you sign up and the pay is not low.
Most people seem to have the idea that you do the six months/1 year for the rating & the hours & then bu**** off; or if you are offered it, join mainline Ryanair where it is much more stable?
Hope this clarifies