[GTALUG] Does anyone use TekSavvy's VoIp?

Hi, TekSavvy supports VoIP by selling you an Analog Telephone Adapter (ATA). I'm figuring out how to open a open holes in my firewall to connect it to TekSavvy's server. I've asked them what subnet their servers are in so I can restrict traffic from the ATA to it. But the TekSavvy techs seem really confused by the question. Have I got it wrong, is it the ATA that makes the connection in VoIP? -- Michael Galea