trying to invite son but "Error! This subscriber is not eligible to join your Multi-Line Account"
I am trying to add my son's prepaid AT&T account to my multi line account that I have with my other son. But I keep getting a error "Error! This subscriber is not eligible to join your Multi-Line Account". There is no other information provided that I can use to try and fix. How do I try and fix this or who do I contact to try and fix? There is nothing special about his account compared to my other son.