UPDATE: Cause of wireless voice outage unclear, some intermittent problems may persist

TORONTO – The cause of a service outage that affected voice calls on numerous Canadian mobile brands, starting Sunday afternoon, remained unclear Monday as carriers said only some customers continued to have problems.

"Teams are working hard to fully resolve the issue and we sincerely apologize to our customers," a Rogers statement said Monday.

Telus, Bell Canada and Freedom also experienced problems with voice service about the same time but restoration began late Sunday.

None of the carriers had publicly stated a root cause of the disruption as of Monday afternoon.

The degraded quality of service included dropped wireless voice calls and an unspecified number of customers being unable to place or receive wireless voice calls.

"Some customers may still experience intermittent wireless voice service," Freedom said in an email Monday afternoon. "We sincerely apologize to any of our customers who were impacted by this issue."

A Telus email statement said its customers began reporting an inability to receive or place voice calls with Freedom Mobile or Rogers customers starting about 2 p.m. Sunday but those problems were resolved about 1 a.m. Monday.

Bell Canada said its customers began experiencing problems Sunday afternoon as a result of problems with other carriers, which weren't identified, but the service issues were resolved around 10 p.m. Sunday.

"We understand that customers at all Canadian wireless companies (including Bell, Virgin and Lucky Mobile) would have encountered problems calling or receiving calls from customers at the affected carriers," Bell said in an email.

Besides their flagship brands, Telus offers the Koodo and Public Mobile services and Rogers offers Fido and Chatr.

Companies in this story: (TSX:RCI.B, TSX:T, TSX:BCE, TSX:SJR.B)