I ran into the same thing. From what I've read, Telegram use to take VOIP numbers, but not anymore. So there are older users who still have their Google Voice number attached to Telegram but new users cannot.
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.
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.