I am looking at T38Modem. A relatively new version is ready for someone to build an rpm for (Mar '07).
But the readme warns:
Q. I try to use T38modem, but after run "t38modem -p ttyx0" I get a message "Could not open /dev/ptyx0: No such file or directory". A. Looks like you don't have legacy PTY devices compiled in your kernel. You need to re-compile the kernel with 'Legacy PTY Support'.
Is this an issue for Centos 4.4 and/or Centos 5?