MTS will still need to approve this for release to its supported devices.
I found this information on Samsung's planned roll-out dates for their Marshmallow (Android 6.0) updates:
Fall 2015 Devices (Note 5, S6 Edge+) Decemeber 2015
Spring 2015 Devices (S6, S6 Edge) Janurary 2016
Fall 2014 Devices (Note 4, Note Edge) February 2016
Spring 2015 Devices (S5, Aplha) in development
If you have an S4, Note 3 or something older directly from MTS chances are high it will not received this update.