I wondered about that myself, recently.
My OBi devices are set up with regular (non-emergency) inbound and outbound calling service from Callcentric (competitor to Anveo). Both Callcentric and Anveo use the same type of VoIP E911 third-party service provider. In my case, I subscribed to 911 service from Callcentric. My devices are setup to use SP1 for outbound 911 calls (SP1 is one of my Callcentric numbers). My OBi is therefore already set to ring on received inbound calls for SP1. Therefore, the 911 service will place a callback to the same SP1 user agent (xx.xx.xx.xx:port) as was used to call 911. It does this by assigning a different 10-digit inbound number to my account, which it displays to the 911 dispatcher. Calling that number will appear to my OBi as an inbound call on SP1. I tested this as follows:
Call 933, the VoIP 911 verification test number. It reads back the special return-call phone number, then it reads back my registered street address and confirms it's set up and active. I hung up, and then I called the special number from a cell phone. It immediately rang my SP1, just as if I called the regular CC number. OBi call history showed it as an inbound call to SP1.
Now, if you do not have a regular number from Anveo, and you are just using it for 911 service, provisioned on, say, SP2, the 911 callback will be routed to your OBi on SP2. What's unclear is, if there is some built-in override for this condition, that will ring the line anyhow, even if the box is unchecked. It wouldn't hurt to check-mark that box. I haven't taken the time to analyze the OBi digit maps to see if it sets up inbound calling to ring the appropriate SPx, regardless, when 911 is being used. I kinda doubt it.
Just my personal opinion. I'm open to other points of view.