I have achieved that two asterisk boxes are registered between them, with iax. One of the boxes can establish calls and receive but the other not, when you attempt with it I have the following message:
chan_oss. c: 682 setformat: Unable to re-open DSP device /dev/dsp: No such file or directory
The number behind chan area oss.c: . I search in /dev and I have no DSP, which might be.