The problem is easy to reproduce: just reducing the children parameter to 1, and openser will not handle a second call (obviously that doesn't happen in Debian, using the same configuration for openser).
I just tested this on FreeBSD 6.1-RELEASE, using openser 1.1.0, and I could place two simultaneous calls without any issues or delays.
As you may know, freebsd has a few different thread libraries. I made sure all my compiles, for everything in and around openser, was compiled with -pthread on the command line.
-mark