Alright, so the site does have fiber to POTs and the busy disconnect supervision absolutely helped. It took it from several bad voicemails a day to a few per week. That being said, the customer still wants it addressed. The voicemail isn't over loaded or nearly full.
Initially I found that Spectrum(provider) seemed to disconnect the calls at 20 seconds if they hadn't been picked up and then the call would enter the IPO as a dead call and you would get the fast busy. So, to try to combat that I set the voicemail pickup time to 18 seconds and that really seemed to help.
The remaining calls that end up in VM as a fast busy seem to be legitimate calls. Their customers will follow up with an email stating that they left a voicemail and the caller id matches what their customers use. So, its not a spam call, its a real call that gets disconnected for some reason and the far end user things they are leaving a voicemail but its just into dead air I guess.
Any ideas?