If you are running a business on Twitter, I don't see a big issue with having a phone number connected to that Twitter account. It doesn't strike me as unreasonable.
Depends on the business, and whether there's a limit to the number of accounts a number can be linked to. A business can quite legitimately have multiple accounts for different departments, brands, or any other purpose. If you can only have one account per phone number, that can quickly become a problem.
Their help page claims a phone number can be linked to 10 accounts at once, but in practice I've found that limit to be 4 or even less. Also, they often prevent a number from being added to more accounts for a few days or weeks even if you remove it from all accounts.
