We are all on the same family plan and can send SMS fine. When we try to send an MMS to our son's iPhone, he will not receive it or any notification of it. Sometimes we get a "failed" on the message. I have the S5, my wife has the Droid Maxx. There are no other issues with the service.

I thought maybe I had restricted his phone at one time due to service limit but if that is true, I cannot find where I changed it.