Here are some documents which may assist. It is strongly recommended that you carry out your own testing in advance of any sale, before committing to using a specific provider:
[British Telecom] (https://www.ipcortex.co.uk/content/image.rhtm/ID-384163img.pdf)
Gamma Telecom
Gamma Telecom (IP Auth)
Gradwell
iHub
Metronet
node 4
Orbis
SipGate
Skype
SofCall
Spitfire
Timico
Tipicall T1 Platform
Tipicall T2 Platform
VoiceFlex
VoIPTalk
VoIP Unlimited
Zen