Sub process

I've designed two separate processes say p1 and p2 and a separate diagram for each of them.
Each one has start and end nodes and also separate business variables and actors for itself.
Any of them executes successfully.
How can I make p2 as a sub process of p1 so that after starting p1 and after executing some tasks of p1 then p2 starts automatically and after finishing p2, p1 continues its running until the end.
PS: when I create a new Call activity and enter the name of the p2 in Sub process name field and run the p1 before entering the p2, p1 runs normally but after reaching p2, call activity fails. Meanwhile when I try to use a subprocess event I can't find any field for it to enter data and configure it.
Thanks in advance