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I moved out of the country and depended on my number being on Google voice to use existing services. I'm also curious how to check if a US number is associated with a VoIP services.

I worked for an international Telecom whose SMS gateway would just broadcast messages to the two other national providers if the number wasn't there's (the other providers would drop messages that didn't belong to them). That was one of my first assignments; writing a task that would check the ported number database and only send the SMS to the correctly ported provider.

I really think this is a terrible guideline. While I understand their concerns, there are plenty of people for which a VoIP number is their only number. Kinda just leaves them out in the cold there.

You can do a fair job with this through number portability lookups; there's a per lookup fee though (although, if you're going to send an SMS, you're paying per message anyway).