I've been a Google Voice user for 6-7 years. It became more important to me 5 years ago when I moved to the UK and ported my mobile number to GV. It's not been very pretty. We were thrown a lifeline with the Google Hangouts app taking over for the absolutely-awful Google Voice app. One of the most frustrating things about GV is that for certain SMS providers, it fails the "is this a USA number test" or "can we SMS to…
I just ran into the "this isn't a phone number" issue with Telegram this morning. It claims my Google Voice (well really Project Fi now) number is a VOIP number, so I can't use it :(.
I suspect this is due to the NIST report about using VOIP/SMS for 2-factor was not secure (which is true. They can be spoofed).
I've also worked for a teleco that, in the case of getting an SMS for a ported number, would just send that SMS to ALL the other national providers in that country. We used relays for the other providers that would just drop the message if it didn't belong to them. My first job was actually to fix that by doing a ported number lookup and sending SMSs to only the provider it was ported to.