Alright I figured this out:
1) Configure Auto Attendant:
Voice Services -> Auto Attendant -> Auto Attendant 1
- Make sure 'Enable' is checked
- Set 'Primary Line' to your default voice service, mine was SP1.
- If you want to require a password, select 'UsePIN' and enter your password in 'PIN1'. Since we're totally trusting caller ID to determine who has access to the Auto Attendant I figure it's probably better to use a password since someone could spoof the phone number.
2) Set the InboundCallRoute to determine what inbound phone numbers can access the Auto Attendant -
Voice Services -> Pick-your-service (e.g. SP1 Service) -> X_InboundCallRoute
- Set X_InboundCallRoute to {(1234567890|1234567891):aa($1)},{ph}
The example above allows the two dummy phone numbers to be able to access the Auto Attendant, if any other incoming phone numbers are detected they'll call you just like they did before. The ($1) is optional - it means if you call the auto attendant but hang up before the prompt your Obi will call you back and allow you to make calls, I guess it's useful if your plan has something like free incoming calls. You can alternatively press 3 at the Auto attendant for the call back, I just prefer hanging up before the prompt.
Hopefully that helps other noobs like me
Mark
