About 18,000 homes in the Napier area have been cut off from the phone network, Telecom says.
A hardware fault at a city exchange has made calls to and from the rest of the country impossible on domestic phone lines.
Phones in Napier, Taradale and Takapau have been affected.
The 111 service has been redirected through Hastings.
Customers in the affected area can call within it, but not to the rest of the country.
Telecom said it had sent technicians to fix the problem but did not know how long repairs were likely to take.
The outage was not affecting broadband or mobile services, Telecom said.
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