Voice and Data networks must have Quality of Service (QoS) enabled to ensure that data uploads or downloads do not rob the bandwidth you need to ensure high quality voice. Below are network diagrams of two options for Voice and Data networks using common infrastructure.
This is a typical network topology for a Microsoft Small Business Server Data network with VoIP phones, and Mission Critical failover between two WAN xDSL circuits.
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.
Please view the voice network diagram for further details.