Residents in parts of four states were unable to call the emergency number 911 on Wednesday night, officials said.
Portions of South Dakota, Nebraska, Texas and Nevada were affected by the outage which authorities had no answers for as of Thursday morning.
Service was later restored in Las Vegas, Nebraska and South Dakota, The New York Times reported.
Most lines were down for one to two hours Wednesday night, according to The Washington Post, although services appeared to be gradually restored overnight.
The exact nature of the outage appeared to vary across states, the Post reported. While some 911 systems reported problems with calls from landlines, others said customers using mobile phones were unable to call in.
The Del Rio, Texas, Police Department said the issue was caused by “an outage with a major cellular carrier,” though they did not name the carrier.
The department noted on Facebook that the “issue is with the carrier and not the City of Del Rio systems,” urging the public to call 911 with a landline or through a functioning carrier if they have an emergency.
In Las Vegas, issues with emergency calls were reported for several hours before police said services had been restored around 10 p.m. local time.
Officers had earlier said they had been able to see when an emergency call was coming from a mobile device, even though the calls could not be completed. Police phoned the callers back to ask if they needed help, the BBC reported.
In Rapid City, South Dakota, police said they were “inundated” by callers trying to see if the line worked.
The outages come amid National Public Safety Telecommunicators Week, the Post reported. The event celebrates the work of emergency phone line workers across the United States.