Verizon Samsung S6 with T-Mobile Sim

Check this out & contact T-Mobile regarding the issues.Also,if applicable with your phone,you may need to enter the proper APN.
http://www.t-mobile.com/bring-your-own-phone.html#compatibility

Compatibility results for: Verizon | Samsung Galaxy S6 (Verizon)

• T-Mobile Network:
  
  Good news, your phone is compatible with our network. However, if your phone is not unlocked, you’ll need to check your warranty and contract with your carrier to see what conditions apply to unlocking your device.

• Network Speed:
  
  Good News! Your 4G LTE phone can take advantage of our Data Strong 4G LTE network.

Bringing over a used phone? You’ll need to make sure the serial number isn’t blocked from our network Check now

 

That's fine,exactly as mine on my T-Mobile branded device.

 

I'd give 'em a call,see if there's something going on with the network in your area & while you're at it,if you can,call from a different phone & have 'em ping your S6/send a refresh,or,whatever they're calling it these days.