Scheduling is a common theme in poly relationships. If you haven't already, sit down with him to find out if he's got regularly scheduled date times with others, so that you don't interrupt. If you think that you can handle knowing, ask him to let you know when he's with someone so that you don't interrupt. Or, work out a system where he simply doesn't answer the phone if he's occupied, and that's okay with you.
There are so many options. Poly is about making up your own relationship structure with those you love (or potentially love!).
How long have you two been seeing each other?
Do some tag searches around here for mono/poly, secondary, and foundations. You may also want to check out the secondary's bill of rights thread to make sure your guy is legit, and not just using poly as an excuse to sleep around.