Atcom have recently built a 1 port version of the IP04, called the IP01. The IP01 is a palm sized Asterisk IP-PBX. It has an ATA form factor with serious CPU horsepower for around US$160 (plus analog module). Ideal for an office or home that has SIP phone extensions and VOIP trunks and maybe one legacy device like an analog phone or Fax. Sure beats a x86 PC with a X100P FXO card for form factor!
The single module can be either FXO or FXS. A full DB-9 is broken out on the back for the serial console – I really like this as it makes hacking easier. The IP01 is based on the open hardware and software IP04, and runs all the same software; including uClinux, Asterisk, Zaptel, Oslec echo cancellation, and the new Asterisk Now 2.0 GUI.
It is totally overpowered with a 400 MHz Blackfin BF532, so it can handle serious DSP work like echo cancellation and transcoding.
With a 256 Mbyte of flash and 64M byte of fast SDRAM the IP01 has plenty of room for uncompressed voice prompts and voice mail, plus all the software you will ever need on an embedded device. For comparison the IP01 is about 10 times the CPU power (faster CPU clock and more importantly SDRAM bus speed) of a WRT54 and has 64 times the flash. Compared to an ATA it has high quality echo cancellation, runs a real operating system (uClinux), and is totally open (hardware and software) and therefore very hackable.