The Comcast "voip modem" is the ATA - which is the line in to your house, in this case. If it has been disconnected from your internal wiring, then the line is dead. For Comcast to continue to provide 911 service, the ATA would have to remain in place.
The Comcast ATA most likely had a cable line coming into it and a phone line coming out. That phone line would go to the rest of your house. What was the phone line coming out of the Comcast ATA plugged into (or possible hard-wired to)? You want to replace the Comcast ATA with the OBi so that the line coming out of the OBI PHONE port plugs into the same thing the line coming out of the Comcast ATA did.