Howto:No ringback tone during transfer

This applies to blind transfer in general:

Incoming SIP call from SIP provider to Waiting Queue in PBX.

Waiting Queue answers the call and plays announcement.

After DTMF from caller, the call is transferred to a PBX phone (blind transfer).

For the time between call transfer and call answer on the transfer destination, the PBX puts the calling party on HOLD.

This HOLD indication must be forwarded to the SIP provider. To get that working you must enable "Interworking(QSIG,SIP)" on the gateway route for incoming calls from SIP provider.

A re-INVITE with "inactive" will be sent to the SIP provider. The SIP provider is responsible to play an announcement (hold tone or ringback tone) to the calling user.

If "Interworking(QSIG,SIP)" is disabled, you will have nothing but silence on the callers side, until the transferred-to phone answers the call!