It wasn’t about the phone
So after a great analysis of the situation, i helped DH to come with a solution about SS holidays.
DH called BM to tell he agrees for the phone during july (whitout explaning the plan to block MIL).
BM said it was great, and that she will tell SS tonight.
DH had SS on the phone, and SS talked about the fact that BM asked how much time he wanted to spend with us this summer, and that he wants to come one week. DH didn't answer to that.
Then, tonight, she sent a message to DH, and if i sum up : she talked to SS. But SS said that he didn't want to come the entire month. Actually, he only wants to come for a week. BM said that she doesn't want to force SS to spend the month, or even 2 weeks because he is physically sick of the idea. She said that SS was really mature for his age, and mature enough to decide what's best for him. She says that she doesn't want to private DH from his son, but that the well-being of SS must be prior to adult desire. That she would adore the idea to let SS spend the month, but that he doesn't want. That it's not an easy decision for her to take, but that they both need to "listen to little SS opinion".
DH is just shocked, and doesn't process the BM text yet.
For my part, i have the impression here to see MIL deepest desire becoming true : SS rejecting his father. The impression to hear her words "in a few time, when SS won't want to see you, you will only have your eyes to cry".
No comment on BM analysis. If my son didn't want to see his father such deeply, i would try to discover what is going on in his mind, try to help him with those feelings, but i would never assume that he is capable of knowing what's best for him at 10.