Use a desktop or laptop computer's web browser, not a tablet or smartphone. Sign into the correct Gmail account that holds your Google Voice phone number, and go here:
https://voice.google.com/u/0/settingsDelete ALL of the linked phone numbers and devices that you no longer use. Write down the remaining linked phone numbers.
ALL of those phone numbers must support a telephone carrier-supplied feature known as "Conditional Call Forwarding", also known as "no-answer/busy forward". Do not attempt to use a number unless it supports that feature.
Contact the carrier supplying service for the linked number(s) and ask them how to turn on (enable) Conditional Call Forwarding, then turn it on by entering the appropriate command on your phone's own telephone dialer (NOT on the Google Voice app or OBi device). For example, if you have a AT&T Wireless phone number, and your Google Voice number is 1234567890:
To enable Conditional Call Forwarding, enter each of these individual call forwarding commands, one at a time, followed by touching Send or Call, hanging up, and entering the next code, etc. Substitute your Google Voice number for the 1234567890 below:
Call Forward if Unanswered: *61*1234567890#
Call Forward if Unreachable: *62*1234567890#
Call Forward if Busy: *67*1234567890#