This is probably the only area that I can call myself an expert in.
Asterisk can be made to build a tricked out phone system of any size at a trivial cost.
Ultimately, this page will be a set of config files, patches and dependencies for building such a system.
- MELP/MELPe - Military grade encryption for minimizing bandwidth consumption
- AMR - What cellphones use
- g72x - What voip phones use
- ilbc/speex - Open source low-bitrate codecs
- Bluetooth will be implemented via chan_bluetooth or chan_cellphone
- GSM will be implemented with a commercial gateway.
- Regular POTS service can be done using hardware
- SIP - VoIP protocol
- IAX - Lightweight VoIP protocol
- Used to switch calls
- Used to translate between various protocols
- Used to perform IVR functionality such as voicemail
- Not all that great in the open source world, but reasonable security can be achieved.