For integrating voice and data, you will need to connect your EPABX systems to your WAN routers, because you want to utilize the same WAN link for transferring your data as well as voice. If your existing EPABX supports IP, then you can connect it to your router's Ethernet port directly. But if your EPABX does not support IP, then you will need to install a special interface card on your router, and configure your router for converting the voice traffic coming from the EPABX to IP traffic.
You will also need to keep in mind that voice has different needs than data traffic. Data traffic is prone to errors, while voice is adversely affected by delay. So you will need to do some special QoS configuration on your network equipments to ensure that voice traffic gets priority over normal data traffic. You may also have to reserve some bandwidth for voice traffic.
This was first published in October 2004