The feature Parallel Ringing – One Number Service (ONS) is used for one person using different phones. Incoming calls are signalled at more than one destination in parallel (simultaneously).

Also a typical scenario is also the use of One Number Service for service deliveries. Depending on who is on duty, the responsible service technician is reachable over one fixed number. If the responsible engineer needs to recall the customer via the SIP device, the server number of the help desk is shown in the customer's display. His own mobile phone number cannot be seen. This ensures that the customer always reaches an employee when calling this number.

An incoming call is blocked by busy, if one station of the parallel ringing station group is busy.

All internal ONS members have the same idle display showing the ONS master number.

Exception: If a CMI mobile was configured as ONS slave, the idle display does not show the ONS master number. The CMI phones keep in idle state their own numbers on display.

For a basic incoming call to the ONS group the call is signalled at all ONS members. All ONS members see the calling party information. After answering the call by one ONS member all other group members are disconnected from the call. The whole ONS group is called no matter whether the ONS master number or a secondary ONS number has been dialed.

If the ONS master is out of service any incoming call to the ONS group is blocked. If one of the secondary ONS phones is out of service the other phones shall work as usual.

For the ONS call to the external number no CFNR is honored; in this case the external call is released but the internal ONS members are still ringing. The purpose of this processing is to avoid that the ONS call only reaches the mailbox of the GSM network although the ONS person is near one of the internal ONS phones. Trunks who cannot receive reliable answer and disconnect signalling must not be used for the ONS call to the external member. The information from the GSM via the CO is not as detailed as within the PBX network (especially in negative cases when the mobile phone cannot be alerted) and therefore it is recommended that the mobile phone is always turned on.