How old is your son? Maybe if he's old enough, you can pay for his own cell phone or iPad that you can use to contact each other. If he's not old enough, you need to tell his mom that you will be forced to get the courts involved, because she is breaching your agreement. Good luck!

Most common divorce orders allow for phone contact or some sort of contact via electronics. I know mine says that my ex (sons dad) has to pay for it. But he doesn't live to far away (30 mins- too close :jawdrop: ) but when my son goes with him for extended stay my ex won't take the calls I make to him or allow it. Go figure! He's a douche bag
I bought my son a phone when he was 6 (now 11) it is a Verizon juke it comes with such great settings that you don't have to pay extra for. I have set it up to only take calls/txt from my phone. He can only make the out going to mine as well (or 911). This is great so I don't have a large bill. And he knows its only for us to contact each other not for friends to blow up. I also have low-jack on it. So I can see where he is.
It makes my ex mad but even when he brought it up to the judge recently, the judge said it was ok. We don't abuse our time. Its pretty quick. It's just his dad is a convicted child abuser (along with me) that I want to know if my child needs me I can be there. Unfortunately it has come to just that. (I said he's a douche)
Anyway, get a phone and if you can low jack it, DO. My ex recently stole his phone and I pinged it and was able to locate it at his dads house. But his dad denied it till I said I have a tracker on the phone and pinged it to his location and I was reporting it. HUH! It miraculously appeared. Oh, the stories I have. LOL Don't be a door mat to her. It's your son too.
Good Luck

If it's court-ordered, take it back to the courts.