Can I use an unlocked AT&T Phone (Galaxy S5) with T-Mobile gear (S2 Watch)?

[legna_c]

I have an unlocked Samsung Galaxy S5 from AT&T. I am now with T-Mobile and recently purchased the Gear S2 through T-Mobile so it has it's own SIM and all. When paired through Bluetooth it works perfect. However, when I leave my phone the Gear S2 says it "failed to forward calls, tap okay to try again." When I'm back in range with my phone it connects and notifications work perfect. It's when I leave my phone behind or walk out of range that I have a failed attempt alert. When I go into my phone settings to manually change the call forward number and settings, I get an alert that tells me "Failed to Read Data. Network or SIM card error." So T-Mobile reps are telling me that it could be because it is an unlocked AT&T phone and I just need to buy a T-Mobile phone. Any idea if there is a way to fix this error without buying a new phone (my current one works perfectly fine)!?