Took a while to dig through forums and trial and error.
Google Voice voicemail dtmf did not work with "Use This Service for a Security Alarm Line"
This setting pushes down the config:
- device webpage/service provides/ISTP profile ?/General/DTMFMethod = "Inband"
If you uncheck, "Use This Service for a Security Alarm Line"
It becomes:
- device webpage/service provides/ISTP profile ?/General/DTMFMethod = "Auto"
and Google Voice voicemail dtmf works.
...and then, after changing it to Auto, your alarm panel probably won't be able to communicate with the central station. It's a crapshoot. The Ademco alarm protocol is very old, and it uses DTMF tones to communicate with the central station. That's why the configuration changes to inband. If you use the OBi as an alarm communicator with Google Voice, you have to dedicate it to only be used as an alarm communicator.