I have a friend who used to work on cell towers and he'd need to do a test call after certain types of maintenance, and might visit a few towers a day to do that work. On the other hand, a particular phone model probably only needs one or two live test calls per firmware release, so there's probably a lot fewer of those calls overall, even if you're at a dispatch center that serves an area including people testing phones.
Cannot have FCC etc. uncertified phone connect to normal networks - that would be a mess.
I’m not aware of any western provider saying “oh, that block of IMEIs aren’t allowed to connect to the network”.
Canadian government shadow-banned sales of a bunch of Xiaomi cell phones, but local providers have no issues authing them onto the network. Can’t say no to that juicy roaming revenue even if it means putting your network at risk.
I don't think they'd block them from roaming if the device has a foreign network sim that's authorized to roam though. Of course, major US carriers basically don't allow their subscribers to roam to other major carriers.