My main concern is what to do when Google kills it.
In general it is much simpler than that. These companies are sending you these SMS messages not from a "normal" phone number (xxx-yyy-zzzz) but from a shortcode (xxxxx). The "from" is a shortcode and only mobile numbers can receive SMS from shortcodes.
So if your number is not a "mobile" number, you might still receive SMS from other real phone numbers, but you cannot receive SMS from shortcodes.
Twilio, for instance, does not provide mobile numbers. Period. So even if you port a mobile number to twilio, as soon as it is theirs, you cannot receive SMS from shortcodes.
Here is a list of carriers that are properly registered to receive SMS from shortcodes:
https://usshortcodedirectory.com/faq/what-wireless-carriers-...
"... short code carriers have arrangements to exchange messages with mobile phone numbers only ..."[1]
[1] https://support.twilio.com/hc/en-us/articles/223133447-Not-R...
The number needs to be non-VoIP. Off the top of my head, Uber, Lyft, Craigslist all require non-voip which means no Google Voice.
I've ran into the same issue with this on Google Voice. Though sometimes I wonder if I could have gotten the best of both worlds by just porting a number from a traditional carrier to Google Voice/Twilio.
Are these kinds of checks only against the number itself? Or is there some kind of dynamic registry?