You can get your GV VM via email. Go to Settings/Voicemail. There is a slide switch to get VM via Email.
I have not tried the following, but it should work.
You can forward calls to the Auto Attendant when the line is busy.
This will stop the call from going to GV VM.
After you get this to work, you can record a custom AA greeting of a busy signal.
Here is a 20 second recording of a busy:
https://www.youtube.com/watch?v=XrmP67QUSUgYou can use the recording multiple times for a longer busy.
This won't work when a 3rd call arrives when the 2nd caller is listening to the busy.
Post back if you're interested and I'll show you how to configure this setup.
You will need to know how to use OBi Expert.