I thought there was some kind of federal thing now that says dialing 9-1-1 has to automatically be processed through the system no matter the keystroke combination for outside access? Something about several deaths or serious injuries due to the confusion of getting to emergency services through the older "Dial X for outside service" systems. No specific memory of the requirement, just a thought rattling around in my head.
I know legacy systems require keystroke combos because they are switching physical circuits to get an outside connection (a la old school "in-house" PBX systems). There's no such restriction on VOIP service.
BRad