It doesn’t sound like your mother in law’s phone calls are harming you in any way — or that they’re harming your husband — so is this just jealousy on your part? ❓ ❓
I know you wrote that she throws her opinions in your face, but if she’s on the phone with him, she’s leaving you alone! 😆 Right? And you haven’t noted that she’s trying to turn her son against you, so, if he likes talking to her, and it’s not interfering with his work or his time with you, I don’t see what the problem is. 😕
In addition, don’t forget that the two of you are LIVING with YOUR parents, so you probably talk to your folks multiple times a day because you’re sharing a house with them. Being with your parents may remind him that he misses his — or his mother may feel left out because her son is living with your folks and not with her.
Try to be a little generous in this situation since it really doesn’t sound from what you’ve written that your mother in law is damaging anyone — she’s just being a little pesky and getting on your nerves. I bet you can find a way to laugh it off. 🙂