I've hit my bug threshold for now, and I'm giving up on DIGITS. Hopefully there is a version 2 coming, and I can try it again later.
I ported my Google Voice number over to T-Mobile and had high hopes of using it to get calls and SMS messages on my computer and tablet in addition to my phone as I had with Google Voice, but DIGITS isn't ready for prime time.
I initially had problems with the native DIGITS integration on my LG G5 after I first starting using DIGITS. Once DIGITS was enabled, my phone always prompted me for the outgoing line to use when making hands-free calls (OK, Google). Even after I worked with Support to do everything possible to disable DIGITS and remove the other lines from the phone so that there was only one line to choose from, I still had to select the line by interacting with the screen. It made it so that I really couldn't place outgoing calls with Android Auto.
I replaced my G5 with a G6 to get away from DIGITS integration. That resolved the hands-free calling problem, but I suspect some configuration settings on my line are still fouled up from the effort made to disable DIGITS. Now SMS handling is too flakey to be useful. Sometimes incoming messages go to my phone, and sometimes they go to the DIGITS client on my computer. There is no real pattern. It is impossible to carry on a conversation, because sometimes the response goes to a different device than the one I sent my message from.
I had too many support calls to count and an engineering ticket open for awhile to resolve the first issue that I eventually gave up on and replaced the phone. I've made several calls on the second issue and have another engineering ticket open on it. At this point, I have put a stop to the frustration by uninstalling the DIGITS client and walking away from it for awhile.
Great try, T-Mobile. I hope you are still working on it.