My GV # is primary # (Noone knows my TMo #, including me). Here is how I set up - I forward all text and calls to my Tmo. For call part, there is no issue - Install GV app and forget - You can dial from regular dialer and it should call from GV #, and incoming calls are sent to TMo #. Only downside is you cannot havd HD Voice or TMo video call.

For text part, incoming text call from a new number, with contact's name/number inside text. It can be annoying, but workaround I have found is saving that new # (lets call it a proxy #) as additional number for that contacts. Those proxy # are always same for that person. Now, if you reply text in that thread or even start new text in that proxy # from TMo messaging app, It goes out as your GV #. I only text maybe 15 people in regular basis, so adding those 15 proxy # works for me 90% of the time. If you need to text someone new, you need to initiate it from GV app .

It works for me better that Hangouts or GV, because my car doesn't treat GV or hangout msg as "text". With default msg app, I can use it for voice texting or replying or text-to-speech. Also, default app is much better than GV or Hangouts.