Generally these tax estimates are provided to companies by whatever tax agency they utilize to estimate what the tax will end up costing them in different places. They can and will be wrong sometimes, and can differ from company to company and specifically with it changing from not only state to state but sometimes county/area code, it’s really hard to lock down a concise answer. If you get a quote for tax estimation and it’s +/- 5% , that’s to be expected, imo.
Yeah, just too many moving pieces. County A in Oklahoma passes a bump in tax or fees related to ip communications, you’re already billing a client based on xyz original estimate, voip company ends up owing more in taxes than they originally thought and realize theyre losing more on you as a client than other accounts (this happens several months/quarters after the increase), they realize, and update the taxes associated with your location.
Someone more affiliated with finance could probably explain it better, I was the tech guy for a smaller voip company but heard enough about specific billing/tax compliance in different states to know there’s like 3-4 different moving entities involved and it doesn’t surprise me in the least when those numbers are off or change quickly.