Wow, um ok so I would make sure that they are different subnets and I wouldn't stop there. I would create separate vlans for all three networks (DATA, VOIP, SIP Trunks) and any other things you are putting across the network, as an added side i would implament some kind of QOS or TOS. I also would not rely on a basic low end router and switches to do the work. you want something that does the work with minimal delay to the aggregate throughput.
I would start from the beginning and draw it out and work it out before actually doing the work, It is notebook time. Take many notes.