
There are quite a different ways to configure a voice and data network, however, the best option is usually to have a separate DSL circuit for the telephones.
This is a typical network topology for a Microsoft Small Business Server network with voice phones, one router and one external, public static IP.
This network diagram illustrates how a voice and data network can be configured so that the router, Mail Server and phones all have external IPs.
A Simple VoIP Network