Republic Wireless mobile numbers often look like voip because it's a MVNO run by Bandwidth.com (a large wholesale voip carrier). It's apparently in the process of being sold to Dish Wireless though, so that may change.
I think the IRS may have access to customer names associated with mobile phone accounts to confirm identity, but not all carriers have identity information for their customers, and I'd guess smaller carriers (or privacy focused carriers, if any exist) may not provide that access.
I think the IRS may have access to customer names associated with mobile phone accounts to confirm identity, but not all carriers have identity information for their customers, and I'd guess smaller carriers (or privacy focused carriers, if any exist) may not provide that access.