IP voice standards
There are standards for VoIP - but you need to search a bit to make sure you get to the right ones.
It helps to have a manual or 2 at hand - because you need a standard you can actually use on the equipment you want to deploy and that works with the voice provider you are looking at...
Voice was 1 of the 1st things standards got to in the electronics world and that didn't stop just because we moved from analog to ISDN to SDH and now to IP.
the Wiki is good place to start
In RFCs you need the ones which are "STANDARD" or sometimes "PROPOSED STANDARD" if the final bits of process havent happened yet.
Some of the CCITT standards cover voice - digitisation of the analog voice stream for example in G.711 which was invented for ISDN and SDH is still used.
There are encryption standards for voice, covering both signalling and the voice "payload" (2 separate options naturally so you can pick whether you do 1 or both) - but as others mentioned that is only an issue if you think your traffic stream might be tapped.
Finally there are interworking agreements for UK VoIP carriers - they cover the interconnections, but they imply what it is easy for a carrier to get from a user device - and guess what - that is what gets used......
anyhow - jumping off place for NICC if you dont just google for UK IP voice interworking and pull the right PDF....