Tone filter and generation

I currently have a RoIP system consisting of several always-on H.323 calls between 2 end devices using Multitech VoIP gateways. One device (A-end) on the end of the call produces a constant control tone at 2100Hz or so. When voice is submitted to the A-end device, the device removes the control tone and submits the voice to the VoIP gateway. On the Z-end, the interruption in tone signifies voice content, so it opens a channel to/keys up a transmitter and sends out the voice.

Unfortunately, the system’s tolerance for the jitter and packetization delay in between these devices is very low. This causes the Z-end device to constantly key up in reaction to the interruptions in tone.

I would like to put one asterisk box in between the streams at the Z-end. It would filter the custom frequency out of the stream from the A-end VoIP gateway. It would then submit the control frequency to the local Z-end VoIP gateway. If the stream from the A-end contained voice, it would pass that voice along to the Z-end without the tone.

Its almost if I want to emulate VAD (silence suppression), but instead of silence, I want to send a control tone during the silence.

Does anyone have an idea of how I could accomplish this with asterisk? Or if someone has experience using Asterisk with old systems with this type of control mechanism, I would appreciate some advice!


was using an air card - hit a bit of turbulence.