Finally got some definitive answers...
Blue bubbles indicate the message is going to another iPhone (or Apple product); green text message bubbles indicate a non-iPhone. With a minor variation that joemikeb explained elsewhere, you need cellular to text from your iPhone to a non-iPhone. [This is just to clarify what the colors mean]
Additionally, undelivered messages misleadingly default to green (with a red text alert), which is what was happening in my instance.
You say you can Message from your MBP to the
same recipient as blue bubbles, which says to me the recipient is on an Apple product, so the problem may indeed lie with the activation on your iPhone. Your link says "you need SMS messaging to activate your phone number with iMessage and FaceTime." For SMS you need cellular. Catch 22?
You don't have cellular so you can't activate??
Yes, and no...the result of poor documentation, or, more precisely, a lousy (albeit understandable) assumption by Apple that every iPhone will be activated as a cell phone.
It turned out that as soon as I turned Messages "On" on my iPhone (And jumped through the associated hoops such as enabling two-factor authentication at iCloud.com, which is understandable, but why as respects Messages?) it was enabled as respects other Apple devices (which took me two and 1/2 days to figure out); "Activation" enables messaging to non-Apple devices and doesn't happen without a cell plan.
I'll guess that joemike's wife's iPhone, which she's using the same way I had hoped to use mine, had been activated as a cell phone at some point in its life, and that that activation carried over after its associated cell plan ended. (I'll note that the Apple specialist to whom I spoke mentioned that potentiality but wasn't sure it was a realistic one.)
Bottom line is that I can message only to Apple devices, which, at the moment, covers everyone I'm likely to ever want to message, and that my iPhone will say "Waiting for activation" forever.
An unrelated issue is that my iPhone was "blocked" from Messages at the outset, but the tech who figured it out and did an ominously labeled "one time unblock" neither explained it to me (I asked, but she refused to give me an explicit response.) nor left any useful notes, so even the specialist to whom I spoke wasn't able to tell me anything more than that there were several possible reasons.
More: New bottom line: Even if I can't message an iPhone by it's phone number (Because I've got a Verizon phone and it's an AT&T phone?), I can message to the recipient's email address, so all's not lost.
