As far as I know, there are three mechanisms for adjusting blood ca2+ levels
1. Kidney excretion/reabsorption
2. Absorption in the gut
3. Storage in the bone.
I haven't studied the effects of phosphate (do you mind explaining this to me, haven't encountered it in the book), but if we logically say that ca2+ goes down then respectively
1. Increased excretion
2. Decreased reabsorption
3. Greater osteoblast activity