Just to start, I would assume you already have this -
TDM switches for current analog voice system, and data switches for data network. Why anyone would move to VoIP (digital voice) and not combine the voice and data network at same time, is basically shear folly!! Moving to digital voice does NOT improve either voice quality or availability. TDM voice at this time in history is like 99% uptime; you will not get that same level with a date network, whether it is one that only carries voice by itself or both voice and date. The major reason most companies move to VoIP is to combine into one network for cost savings (and so the old voice people can now retire).
Now if you are going to put voice and date on the same network, along with the other 99+% who have moved to VoIP, then you need to take precautions to make sure the voice traffic is handled properly, primarily via COS/QOS. Make sure you implement this correct and through out your whole network.
My favorite line to traditional voice people when moving to VoIP (more true 10 years ago then now) was "we are now going to teach voice people what downtime is!"
Good luck. Also I do not think anyone at Juniper will mind you purchasing twice as many switches for separate voice and data networks -