Depending on the destination I call through our ISDN gateway the destination returns an alerting message and after that a busy signal. But depending on the provider I call the alerting signal can be missing and the provider directly returns a busy signal.
The ISDN gateway translates these signals into SIP messages. In the working case it first sends 180 RINGING and then 486 Busy here. In the other case the INVITE becomes directly answered with 486 Busy here withouth 180 RINGING. When there is no 180 Ringing before 486 Busy here the Polycom simply drops the call WITHOUT playing the busy signal.
Is this behavior intended? Is there an option that I can set? I'm using Firmware 3.2.6. Has this behavior changed in later firmwares?