It's easy and cheap to determine the original carrier (or its sucessor) for a US phone number. It costs money to do a porting lookup to determine the current carrier.

Most of the reason to deny voip users is that many voip services give phone numbers away like candy and then those phone numbers are used to abuse other services, so checking the original carrier tends to be enough for abuse screening.

Some use cases want more though. Banking KYC has some back channel to get subscriber identification or be alerted when ownership changes; those institutions may be willing to pay for current carrier lookups and deny usage of numbers where they don't have a back channel to the current carrier.

In the US, I belive there are three number categories in the NANP porting database (wireline, cellular, and VoIP), and SMS senders can definitely tell, even though it might take a while (presumably there's a lot of caching going on).

If you're lucky, the service you care about only validates at number registration time, not at text sending time, and you can get away with it indefinitely, I suppose.

I thought that too but many carriers around me don't allow porting any VoIP-using number back to cellular. (Not sure if you were making a distinction between landline and cellular)

Unfortunately that means that my cell number which I wanted to temporarily park into VoIP while abroad is now permanently VoIP.