The Communications Workers of America made the announcement after the existing labor contract just before midnight Saturday.
CWA spokesman Al Kogler said in a statement that negotiations will continue and workers will remain on the job during the extension.
Kogler has said the union opposes a proposed increase in health care premiums and wants to bring more jobs back to the U.S. The union had authorized a strike in the event that a new deal couldn't be reached with the Monroe, La.-based telecommunications company.
The workers involved are in Arizona, Colorado, Idaho, Iowa, Minnesota, Nebraska, North Dakota, South Dakota, New Mexico, Utah, Washington, Oregon and Wyoming.
CenturyLink workers in Montana, who are negotiating a separate contract, agreed to the extension.