In the OBi device web interface, go to Physical Interfaces, PHONE Port, and see if DigitMap contains the string |**8(Mli)| (note the bar character separators) and then see if OutboundCallRoute contains ,{(<**8:>(Mli)):li}, (note the comma separators). If those aren't present the **8 won't work. Both should be part of the default strings.
Also you should be able to press # and get dial tone directly from the connected line - does that work?