This is especially problematic as Twitter doesn't allow people to use numbers from Google Voice and other digital phone number services.
If your justification for requiring non-VOIP numbers is "it decreases the number of bots on the service" and the service is overrun with bots, clearly requiring non-VOIP numbers hasn't done much of anything to decrease the number of bots.
Is it a shared number?
iirc they had a guarantee that the number won't be reused for the same service for at least 6 months from order.
Untrue.
Source: my Twitter account uses a Google voice number.