Small VoIP Carrier has implemented STIR/SHAKEN, but blocks callers with Google Voice issued phone numbers. (NOTE: Subscribers of this Carrier have the option to enable/disable STIR/SHAKEN so the problem can be avoided if the subscriber is willing to live without it.)
The Carrier says that one of the STIR/SHAKEN data headers that Google is sending is wrong (they say it involves the second header, but I have no detail), and so they block these numbers. However, the Google numbers work with other Carriers that they have been tested on, such as Verizon. The Carrier didn’t know why that would be the case, but guessed that - aside from some other Carriers not having yet implemented STIR/SHAKEN (big ones were supposed to have done so by now) - maybe with the incorrect header the other Carriers have simply chosen to assume that the call is legitimate rather than defaulting to block. They weren’t optimistic about Google responding any time soon in an attempt to resolve. That of course begs the question if the Carrier itself is doing something wrong, but no joy in going that route.