I didn't even randomize mine, it's literally the same greeting every time and a fixed extension number. Just the fact that it exists as a roadblock to listen to, comprehend and put in some DTMF tones seems to stop 99% of the scam calls.
I'm guessing most scammers don't even hear your prompt. I have a simple, short, "leave a message <beeep>" prompt and the vast majority of my scam voicemails go:
