[asterisk-dev] DAHDI analog: Monitoring call setup by backward audio connection

[Pavel Troller]

Hi!
  I'm playing with chan_dahdi/sig_analog.c, I'm adding a special signalling
mode for a custom hardware (E1 channel bank with FXS and FXO ports) and I would
like to be able to listen to the call setup on the FXO port (i.e. FXS
signaling) immediately since the line seizure. I've discovered that calling
analog_set_dialing(p,0) opens the audio path, while the same call with 1 keeps
it closed. What I would need, is one-way path - backward only. Reason for this
is that the channel bank already transmits the audio, so I would like to hear,
what's happening on the line (monitor dialing etc), but because there is no 
regular phone picked up on the remote end yet, the line impedance is unmatched
and a strong echo appears. The forward speech path should be opened just after
the call is answered (which I can detect from the channel bank signaling). 
Is it possible by just a simple call similar to analog_set_dialing() ?
  With regards,
    Pavel