This tutorial will instructs you to configure Google Voice on your OBi ATA without using the OBiTALK Portal. Contributions to this were made by naf, RonR, and jsolo1.
NOTE: Since late 19 July, Google Voice is no longer working on OBi1. You can build Asterisk server, or switch to another VoIP provider, or buy OBi2.
Since you cannot downgrade new stock OBi2/OBi3 firmware, if you want to retain the possibility of downgrading, you need to use third-party firmware and configure it manually.
Before you start:
If you don't have the right firmware, download and install a supported firmware before you continue.