Unless you're doing a dip of the number against proprietary telecom data sets, you have no idea if the number is a "VOIP" number, due to North American number porting laws, you can take any number that was a "Verizon landline" or whatever and move it to a VOIP provider that can overlay SMS capabilities on it.
Even if you dip and see that it belongs to a VOIP provider, it's a completely legit use case for some to own their phone number through Bandwidth, Twilio, Telnyx, Messagebird, whatever.
There are DBs that can get you that info. Some even tell you when the number was ported which is useful to catch mobile number takeovers. Things have moved beyond NPA/NXX lookups.
Of course, that’s what I was referring to. The consumer still has to subscribe to those data sets, keep them updated, and understand which lesser-known company names are “legit” telecom providers (as many large providers are non-household names and have VOIP offerings) vs whatever kind of VOIP provider he feels he needs to protect against.
My point being that if he’s doing it right, he’s probably spending more time and money than it’s worth, and if he’s not, he’s banning legit users for the crime of not having a big-4 provider.
