Asterisk hardware recommendations

This page and it's siblings suggest some configurations of hardware and software for setting up an Asterisk based PBX. Most of these are based on running configurations in various organisations today. Scaling an Asterisk system isn't easy, there are not simple answers. Next to the samples below we suggest you read about capacity considerations when planning and dimensioning an Asterisk system.
There are a number of VoIP providers that run Asterisk offering IAX connectivity (often with the SIP proxy SER in front of it) . FreeWorldDialup (FWD) and nikotel.com for example employ Asterisk for PBX functions like voicemail, conferencing etc. Quoting Rich Tehrani:
"Currently you�ll find Asterisk (or * as it is referred to frequently on message boards) in corporations where the IS staff is enterprising enough to set this type of telephony server up. You will also find it in numerous CLECs that won�t share this fact with anyone as it is a significant competitive advantage."

Quoting Mark Spencer (in Saarbruecken):
(Transcribed, answering a question about known larger deployments)
One of the implementations that we know about 800 Pingtel IP phones. For busy operators these JAVA-based phones are not ideal since they require regular reboots. Also Vonage is using Asterisk and has made a lot of contributions to the project, they are using it for voicemail. There is also a large analog installation to be found at Frys Eletronics, which is a large US retailer.

Quoting Stefen Sokol (April 2004):
There are carriers using Asterisk to terminate thousands of lines. NuFone has a data center with 80 Asterisk servers in place. These installations require a bit more engineering than the typical PBX server, but the system does scale to extremely large systems.

Francois Lambert, Atelka, Inc:
We are running a call center with 4 Asterisk servers (10 T1 + 1 Channel Bank). We have internally 240 agents using IAX Softphone with uLaw codec.
We have 200 inbound agents and 40 outbound agents. We know that our architecture is scalable to go up in the 1000 agents and more.