Howto:If I tie in a gateway into an existing trunk line, do calls from ISDN to ISDN consume DSP coder channels?

FAQ:

If I tie in a gateway into an existing trunk line, do calls from ISDN to ISDN consume DSP coder channels?

Answer:

Depends on your "Relay" config and the firmware Version you are using. Generally speaking, if a call is routed directly between 2 PRIx/TELx entries, then it does not consume any DSP channel. However, if it is first routed into the PBX and then (e.g. by using the EXTERN interface) to the ISDN again, then it will consume a DSP channel in V5. You can work around this by disconnecting calls coming from the EXTERN interface with a "no channel available" cause code. This will return the call to the relay routing table which will try to re-route it then. A bit tricky, but works.

In V6 though, calls from ISDN to ISDN will be detected and handled without a coder/DSP channel, even if it is routed through the PBX or even an external gatekeeper. To enable this direct call routing, you must set the 'Enable PCM' checkmark, at the VOIP-Gateway interfaces.