Sorry to hear that you are having to deal with this. I see it as a form of torture - I am still triggered when I hear a ring tone like my old cell phone. I let it go on far too long and I hope you can learn from my mistake.
I urge you not to pick up the phone. Ever. If you need to communicate before he is able to do so like an adult, then perhaps consider a lawyer. If you pick up the phone after 20 calls, you just teach him to make 20+ calls next time.
You've a big lesson in boundary setting (and maintenance) here - and it's one of the most important.