I also have the same problem...it is frustrating.
i have an unlocked AT&T Gear S ,
i setup account with Tmobile, and it worked, i have had it about 2 months, all OK, until..
i took out SIM to test with my unlocked IPHONE 6+, (i have been thinking of switching all my phones to Tmobile) it also worked OK,
Then i swicthed it back into the Gear S, and the trouble started (unless it was a conincidence),
'Phone not allowed MM#6', it tried everything.
1. reset Gear
2.rebooted thru the gear boot menu
-i called Tmobile, they did not have answer,m they asked me to try a new SIM, which i did online and updated account with new SIM,
3.Tmo said maybe not complety unlocked to contact AT&T but i doubted this,
4. I tried a freinds SIM from his Iphone 6+ (on Tmo) it worked great in my Gear Sm and my 'unlocked Ipphone 6'
5. i tried my Gear S SIM in my Iphone, message poped up 'Phone not allowed' (this time no MM#6)
6. called Tmo again (maybe 3rd time) suggested another new SIM card, they everything on there end looks fine, they checked imei, sim card etc, all OK.
i told them from what i have read the forums mentioned a network problem, not a phone problem.
They connected with Samsung, they said they can try reflashing the Gear S, if i send it in to them,
7. i called Tmo again, and i spoke one rep who , GAVE ME A CLUE TO PROBLEM.. he found a note concerning the Gear S,
it said if the user puts the Gear S SIM cards (wiith the 'wearable plan') into another phone, it will block the account until the user reinstalls SIM into Gear S.
8. if this the problem, iit is not reseting itself when SIM is back in Gear S.
9. Not sure what to do next.
the last Tmo rep told to go into Tmo store, and they will try another new SIM, and ask them to try new SIM and my in another Gear S, but it seems no tmo stores have Gear S on hand, only non workin display model.
i Did try there suggestion, same. Mm#6.
10. One idea may be to close the whole account and open new account for the Gear S.
did anybody have luck with this problem?