So I'm trying to help a friend switch from Sprint to prepaid T-Mobile, but his Sprint number cannot be ported. His number can be ported to other companies such as AT&T but for some reason is incompatible with T-Mobile. I had the idea of porting his number to AT&T for a month and then porting it over to T-Mobile. In theory, this sounds like it would work but I'm not sure and I don't want to get him to switch to AT&T if it can't be ported to T-Mobile anyway. He is getting an unlocked GSM phone so he can buy a T-Mobile SIM card when he wants to switch.
Does anyone have any experience with this?