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.
Current version doesn't do GSM. Mainly because I have no idea how to do it.
- MELP/MELPe - Military grade compression for minimizing bandwidth consumption
- AMR - What cellphones use
- g723.1/g729 - 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
- IAX2 - Lightweight VoIP protocol
- At least a P4 2.4 2GB server with a focus on reliability (solid disk, passive cooling if possible) $1000
- Backup server/load balancer $1000
- Half decent switch $300
- Digium AEX2460E Analog Card 6x4 FXS + Echo can $1800
- g729 licenses $100
- 20U Rack $500
- Rackmount UPS $500
- Far end server colocation $400/month
- Existing network/Internet
- Cool/dry/secure area
- ~1000 watts available power
- Used to switch calls
- Used to translate between various protocols
- Used to perform IVR functionality such as voicemail
- IAX2 has an aes128 option
- Bluetooth (cough) security
- Not all that great in the open source world, but reasonable security can be achieved.