Phosphate Oral Saline Laxative Question

I'm having a lithotripsy done tomorrow and was told to "clear out" to up my chances of breaking up the stone. I've taken 4 Dulcolax as directed and nothing has happened (took them over 24 hours ago). My Dad told me to take this Phosphate Oral Saline Laxative that he uses and he says it cleans him out within an hour. But, I wasn't sure if it was safe or effective for Roux en Y patients. Is this something I can take? If not, any other solutions?

I find Milk of Magnesia works really well - the phosphate saline laxative should work also as well as citroma or mag citrate (maybe try half the bottle). I am an RN in the ER as well as having surgery - I use good old MOM