Having your own office phone system can be huge asset, but commercial systems are often limited or expensive, and the free solutions have their rough edges. These are my notes from re-building our office PBX. Hopefully they'll save someone some time.

• VoIP is much cheaper than landlines for international calls. Afghanistan costs $0.30/min !
• You know how your browser slows down when your downloading something big? Well, you can imagine what that does to voice trsaffic! Use a separate low contention internet connection, or an office router that can give VoIP traffic priority.
• Asterisk [open source phone switch s/ware] has its flaws, but it is the defacto standard. (FreeSWITCH is more programmable, and scalable, so it will probably take over in the very long run.)
• ISDN is a bit anarchic:
• You'll need to get details of your protocol [point-to-point, or point-to-multi-point] and your TEI [could be set value, could be auto-negotiated]. If you get these wrong, you will not get an intelligible error message and you will be very confused.
• With BT, Caller ID is a paid for option, you'll have to remember to buy :-S
• You will need to read log files and install RPMs to get your phone PCI card to work. If you can't do this, you should buy an appliance.
• Trixbox [a Linux distro with Asterisk and a web UI] is turning "evil", e.g. forking OS projects, putting all the juicy features into its paid-for Pro version. But it's still the easiest turnkey system available. Which is not to say it doesn't have flaws:
• Call recording breaks silently if you choose an unsupported (but still listed in the UI) [not GSM or WAV] audio codec.
• Internal codes, for things like customer queues, can break out and get sent into the phone system if you're not careful with your routing.
• To record DISA [dialling into your phone system with a PIN] calls, you'll need to set the DISA call context to an extension, like 333@from-internal .
• If you're not American, you'll probably need to pick up some sound files from here.
• Obtaining international virtual phone numbers can be cheaper than you imagined. A US/Canada toll free number is just $0.99/month, a Tokyo number is $14/month.
• Non-US/Canada toll free numbers aren't a stable market yet. Most end-user suppliers are just resellers for AT&T/Sprint and Voxbone. So pricing can vary dramatically for the same services.