The City of Calgary is investigating a technical problem that may have prevented some callers from connecting with 911 early Monday morning.

While officials said they still don't know exactly what went wrong, full operations at the city's Public Safety Communications centre were restored by 6 a.m.

The problem started about 2 a.m., said PSC Manager Curtis Brochu.

He said staff immediately activated backup systems but there was up to a 30-minute window when emergency calls may not have connected.

Where information was available, emergency communications officers called back those who had tried to get through on 911 but those callers did not require further action, said Brochu.

"While many of our calls even at that time of day are misdialled cellphones, we are concerned some legitimate calls may have experienced a delay in getting through to the PSC," Brochu said in a news release.