Problem with mms

I have an unlocked Lg g8x from SPRINT. It works great except with mms. Strangenly, I can send and receive mms to some people, and can not do it with other people. In the latter case the message is hanging and I can see sending... , but this sending never ends. This happens even with other clients of Videotron.

I played with all settings, and cleanings, nothing worked. I even put the phone to factory default , that did not resolve the problem.

As a test, I put my sim card in another cell phone, and everything works properly, So, definitly a problem with Lg phone.

Any suggestion welcomed.