My Asterisk PBX Server is built out of an old converted HP Thin-Client, it is configured as follows:

• 1Ghz AMD Geode CPU (Athlon T-Bird core)
• 512MB RAM
• 8GB SanDisk Extreme IV Compact Flash Card
• Onboard Realtek 10/100 NIC
• PCI Riser Chassis (modded with Molex power delivery for FXO/FXS card)
• PCI E1/T1 card (tie-trunk to my Avaya Definity via 24-channel T1 link)
• This connects a few SIP endpoints as well as voicemail for the Definity

I have run this Asterisk box for well over a decade now, it’s been super reliable, though it’s probably time to start thinking about replacing it with something a bit more modern.  It started out with a PCI TDM400 FXO/FXS card, but I replaced that with a T1 card when I got the Avaya Definity PBX, other than that, its configuration has been pretty much unchanged since I put it together back in like 2009.