The unspecified vendor worked with CenturyLink to resolve the issue "as quickly as possible," the company said in a statement, adding the outage was "not related to CenturyLink's network."
"CenturyLink continues to work with the 911 centers to ensure that all issues have been resolved," the telecommunications company said.
A CenturyLink spokeswoman did not immediately respond Friday night to a request for additional details.
The Washington 911 outage was resolved about 7 a.m. Thursday, the company said.
A similar outage in parts of northwest Oregon lasted more than two hours and was resolved about 3:30 a.m. Thursday. That outage affected about 16,000 phone customers in Lincoln, Tillamook and Yamhill counties. CenturyLink said that was caused by a maintenance issue.
In one case early Thursday, an Everett, Wash., woman said she made 37 calls about an intruder breaking into her home and had to drive him away herself with a knife.
The failure was surprising and distressing to Kurt Mills at the SNOPAC center in Everett that dispatches for three dozen police and fire agencies.
"I've worked throughout the country for 25 years, and I have never seen anything like this, never- for a whole state to go down," Mills said. "How that can happen to an entire state is shocking."
Alicia Cappola told Seattle TV station KIRO (http://bit.ly/1iDb2T9) that she called 911 at least 37 times about the intruder, but couldn't get through. So she armed herself with a knife and confronted a man who crawled through a window. He ran out of the house.
About an hour after her first call, she reached someone who dispatched a police officer. The officer took a report about the incident, Lt. Robert Goetz said.
Dozens of dispatch centers in Washington state were at least partially unreachable during the outage. They are all tied together because the system is operated by CenturyLink.
The failure, especially with the Everett woman, distressed the dispatchers.
It "must have been terrifying for her," Mills said in an email. "It truly pains me to think of what she went through and all of us at SNOPAC are tremendously relieved the outcome wasn't worse."
The state emergency phone network is designed to be resilient with redundant backups, he said.
"It goes without saying this is not acceptable and requires an explanation and concrete steps to ensure it does not occur again."
The Washington Emergency Management Division also wants to know what went wrong, spokeswoman Wendy Freitag said.
Freitag did not immediately return a phone message left after business hours Friday seeking comment on CenturyLink's explanation for the problem.
The center in Everett did receive some cellphone calls and voice-over-Internet calls during the outage, but dispatchers realized something was wrong because of fewer calls and a call from a medical alarm company that said a client was unable to call 911. Dispatchers suspected about 20 percent of the calls were not getting through, Mills said.
The center dispatches for 36 police and fire agencies and typically handles about 1,600 calls a day, Mills said.
"We knew there was a problem and initially thought it was us," he said. They called other dispatch centers and realized it was a much bigger problem.
"I expect CenturyLink to be forthright and taking steps this won't happen again," Mills said.