You don't need the ## prefix, just an 11 digit number starting with 8.
You will need to make the following configuration changes using OBi Expert.
IP Phone -> Phone Settings -> DigitMap
Add the following AFTER the beginning parenthesis: (Mli)|
IP Phone -> Phone Settings -> OutboundCallRoute:
Add the following to the beginning: {(Mli):li},
Line Interface -> Line1 -> DigitMap: (8xxxxxxxxxx)
Now dial 84805551212
Be aware that some people experience an echo or hear unwanted tones using OBiLine.
Edit:
You can use the default configuration without any changes to route calls to OBiLine.
Dial the number like this: **7084805551212