An outage intermittently affected 911 calls and back-up systems from about 9 p.m. Wednesday to 4 a.m. Thursday. Dispatchers could see the location and telephone number of callers but could not hear them.
Dispatchers immediately sent officers to the locations and telephoned callers back on cellular and land lines.
Officials did not know what caused the outage. The Durham 911 center has asked provider Positron Public Safety Systems to conduct a full investigation.
Poistron provides services to more than 2,000 emergency communications centers.