This is not an accurate statement because VoIP is just a transport. Real carriers use VoIP for this, it's not about whether or not it's delivered by VoIP but who the Number Authority really is. ILEC/CLEC carriers like ATT, Verizon and even Telnyx/Bandwidth are trusted due to the amount of oversight and regulation along with own infrastructure. All my numbers are listed on Bandwidth's OCN so when verification services do a look up it passes because the number isn't classified as VoIP, it's "landline" or "mobile".
There was a point where Interconnected VoIP Providers could easily register as Number Authorities have have NPA-NXX pools assigned to them. That is much harder now, the rules have changed about it due to fraud and other problems. However, a carrier with no infrastructure like GoogleVoice or TextNow i.e. "Over-the-Top VoIP Carriers" are classified as such. They are able to meet the requirements to be a Number Authority but their pools will be listed as "VoIP" vs "landline" or "Mobile".
When these services look up the number they are looking for the current OCN/SPID, all over GoogleVoice's and TextNow's OCNs are automatically blocked, for example. They are going to look at the NPAC, LERG, RND and reputation of the number. If that number lives on a certain OCN/SPID or has a wonky history it will be blocked.
So Bob's VoIP Shack, who uses Bandwidth, has no issue with SMS verification but Larry's VoIPText uses TextNow which will have all sorts of issues as it will be blocked all over the place. Two voip based companies, two completely different results due to upstream connections.