I asked my husband if he could help you, this is from him-

Are you talking about an airport basestation? If so, that should function just like any other router. If you can't plug a cable from that into your VOIP device then you won't be able to plug a cable in from any other router. If they gave you directions for a router setup I'm not sure why they wouldn't work for your airport basestation. You might want to give their support line a call.

If you're saying that you're using an airport extreme card in a computer that is plugged into your modem to share a connection wirelessly, that's a whole other situation...

Might help if you could explain your setup a bit more...