Why do my ISDN-systemphones show the wrong or no time at all.

ISDN-systemphones receive the time/date from the PBX.

If the PBX operates using an external ISDN line, time and date is automatically set after each ISDN call. However, if the PBX operates using only an external VoIP- or analog connection, time and date must either be configured manually or automatically set via NTP-server (internet access or a local NTP-server required).