Although busy-on-busy is enabled, additional incoming callers get the normal ring-tone and no busy-tone. What could be the cause?

Under the following conditions the PBX can signal the busy-tone for addional incoming callers:

  1. The external call is distributed to only one subscriber; call-waiting must be disabled. Also no parallel call or call-forwarding on busy must be enabled for the internal or external number.
  2. Busy-on-busy is enabled for the external or internal group-number (depending on the PBX). In current systems the maximum number of connections before the busy-tone is played back can be set.

 If these conditions are met, additional incoming callers get the busy-tone. If it doesn't work it is not the PBX's fault anymore. What could be the cause now?

  1. The PBX is not operating directly on the public line and might be connected to the internal ISDN-port of a VoIP-router for example. In this case the PBX is signaling busy to the router. The router on the other hand only recognizes a busy device and eventually calls other devices (depending on the router's own call-distribution). Here it should be checked whether the router also provides a suitable setting or if the PBX can be connected directly to the public line without router.
  2. Another possibility is that another device is operating in parallel to the PBX (e.g. an alarm-system). Here the PBX signals busy to the provider but the other device is available for incoming calls and therefore rings; the caller gets the ringing-tone, not busy. In this case it should be checked if it is possible to disconnect the parallel device from the public line and connect it as internal subscriber to the PBX or the device must be set to not react to incoming calls. For further information the manufacturer of the device should be contacted.

How can I verify that the PBX is correctly signaling busy?

The PBX's call-distribution should be set to signal more than one subscriber, busy-on-busy is enabled or set to "from 1st connection on" (depending on the PBX). For the first incoming call all the configured subscribers ring and one of them takes the call. For the second incoming call no additional subscribers should ring. If they do the PBX's configuration should be checked. If the caller gets a ring-tone but the PBX's subscribers don't ring, one of the above described examples applies. If the caller gets the busy-tone, the function is ok.