CALGARY - City officials in Calgary are investigating after some callers were not able to get through to the 911 emergency phone line.

The problem started about 2 a.m. Monday morning and staff notified Telus immediately about the disruption.

The backup system was activated but for about 30 minutes emergency calls couldn't get through.

Calls to the non-emergency lines of the Calgary Police Service, Calgary Fire Department and Emergency Medical Services were also impacted by the outage.

The system was not fully restored until 6 a.m., and when possible, operators called back to the numbers that had tried to get through to 911 during the outage.

Officials say they don't know exactly what caused the disruption, but they continue to investigate.