At this stage, sale the number to a security researcher for $50, and change the number.
They will make their money back and some, and you'll get compensation.
Then sue Google with the money.
They will make their money back and some, and you'll get compensation.
Then sue Google with the money.
Not a great one as the problem is Google rather than the callers, but I guess you could also have an immediate route to voicemail with the message "this is not, repeat not, google".
It might work pretty well, Google can choose to say oh yeah, that's about right, here's £300 or they can make this into a big deal whereupon their in-house legal team are immediately going to spend far more than £300 just showing up and making their position in court, and they might well lose if you tried to get it fixed and they were useless.