I've had this problem that some / sometimes Google Voice call would go straight to the GV Voice Mail. Part of the really goofy part was I would then get a 'message waiting' signal on my very old caller id box and my phone's message light indicator light would flash - indicating a VM was in que.
It was very scary and didn't seem to have any rhyme or reason. Sometimes the same caller would get through or not.
And then the discovery.
*77 Anonymous Call Block / Rejection was activated.
So incoming calls deemed anonymous would get an immediate busy tone and sent straight to GV VM.
The immediate cure is *87 to deactivate ACR .
As for same caller getting rejected or not - was from a large company were their outgoing calls could be routed differently - sometimes with caller ID, sometimes without.
FWIW, the root cause is the immediate busy, without any other notification to the caller that they are being rejected for lack of a caller ID. There is no way for them to know why they get a busy and then VM. That they might be able to complete the call with *68 (one time) or even *82 (Persistent). Back on my old classic Verizon, there was a message like 'This customer does not accept calls from unknown callers' and then hung up, without giving a busy or going to VM.
I will post a suggestion for a new feature for ACB / ACR