OK. This topic intrigued me so I just did a little testing...
I recently migrated a land line through Tmobile on an old iPhone and then on to Google Voice (GV). Since the number was in an iPhone for a day or two, the number showed up as a send/receive option in Messages. Then, I moved it on to GV, set up with an
obihai device and now have free land-line-like phone service!!! All good there... even surprisingly good!
Messages send/receive options still show the number even though it is now "at" GV for about its third week or so. I switched "start new conversations from" that number and tried a test text to someone with an iPhone. It worked. Tried sending some random emoji. They went too. Then tried a picture and it went too. Confirmed receipts of all 3 with no issues.
I only did this landline-Tmobile (via iPhone)-GV migration in the last couple of weeks, so maybe some Messages server still sees some kind of valid phone number at Tmobile (???) leading this to ultimately fail at some point. But for now, it works fine (and I'm actually surprised it
IS working in Messages).
Key Note: I did not get a new number for that iPhone. In other words, I just used an old iPhone to facilitate the migration from land line to GV (because that seems to require going through a mobile phone to make it to GV). So there's no confusion about Messages working with a new iPhone number because I don't have one. The number is definitely "at" GV, as I've used it many times now for both calls from and to it via that obihai device... and in the Hangouts app.
I'm just surprised that this worked and wonder if it will keep working beyond some point where maybe a Tmobile server interacts with an iCloud server conveying that Tmobile is no longer providing iPhone service. I can imagine maybe a monthly process where discontinued iPhone service updates might be shared with Apple to squash using Messages this way. But, if so, texts still work just as well with the Hangouts app, so that can be the fall back option if some iCloud bell does toll at some point.
Key Note #2: I see some people saying that they don't have the number in their send/receive options. I wonder if a necessary step is like a ping? In other words, WHILE a phone number is actually on an iPhone, text at least 1 message from and/or to it- as I did- and THEN migrate it on to GV. Maybe Apple's servers need one valid text from a new iPhone number to verify it as an official iPhone number??? And then, even if it moves on to GV, maybe Messages/iCloud no longer "cares" (or perhaps catches it later)?