Unless I misunderstood the question, its not the phone system, or switch, that controls access to your local Public Safety Answering Point (PSAP), but rather your service provider. VoIP systems connect to the PSTN and like any other PBX or KEY system, access the same exchange. Now, if you are using a VoIP provider, like Vonage or the like, THEN you have something to inquire about.
John Lever
Telecommunications
Richland School District Two
Columbia, SC