Something like a Shuttle PC is ok. It’s not so much the size I have a problem with, it’s reliability. A PC has fans and hard drives and it’s designed for maintenance. Changing fans every couple years, blowing the dust out, re-seating boards etc. is no big deal. Cases are designed to be on the floor or on a desk so they are easy to get at. A traditional small business phone system is designed to be hung on the wall and forgotten about for 10 years.
It is a VERY easy thing to use off the shelf hardware, load Asterisk@home or whatever, make it work beautifully, put the PC in some corner on the floor and say, I’m done. If your just tinkering or running a semi-serious business where you are also your own tech support then that is just fine. You can live with having a few problems every year or two. However, if your going to be selling this as a replacement for a traditional small business phone system then it better have the same reliability.
Yes, I did see that last addition about the soekris on the asterisk wiki board. That is just one person and he is using a PRI card and FreeBSD. Several others seem to think there is enough horsepower for a TDM04B and serveral extensions all going at the same time. It’s still up in the air a bit but the author of Astlinux seems to think there is more than enough horsepower.
A hardened barebones system is not completely out of the question. There are relatively simple and cheap things I can do to make it more reliable such as RAID 1, UPS, redundant power supply etc. and that goes a long ways. In some instances it might be ok but if your going to go in and sell someone on replacing something that hangs on the wall and has worked flawlessly for 10 years then you should be able to do the same. There are a few desktop motherboards out there that can take a Pentium M CPU. With a big enough heat sink I don’t think they need a fan so that is another incremental thing that could be done for reliability. Of course, the cost starts to creep up so… It would be nice to have something like a Soekris because they are very cheap and very reliable. My 2 main requirements.