Customers on Telecom's XT network in much of the country were left without calling, text messaging or mobile internet after a breakdown yesterday.
The $600 million network was out of action between about 4.30am to 5pm because of a fault affecting a Christchurch component.
Customers from Taupo south were affect and Telecom received a flood of complaints.
Technicians worked throughout the day to repair the fault.
Although the company first forecast the network would be restored by 10am, it was not up and running until 5pm.
More than 100,000 people may have been affected.
Telecom told customers needing to contact emergency services to use landlines instead of their cellphones.
A spokeswoman said: "Until service is restored, Telecom recommends that any XT customer experiencing issues attempting to contact emergency services on their mobile should immediately use a landline instead."
Telecom retail CEO Alan Gourdie acknowledged the significant effect on customers.
"We apologise and deeply regret any inconvenience our customers experience due to the interruption of XT services.
"We know how important mobile services are to our customers.
"We do not expect our customers to pay for services they have not received, so we will advise as soon as we're able how we will manage this," Mr Gourdie said.
Telecommunications Users Association chairman Chris O'Connell said that although his sympathy went out to Telecom, "we feel more sorry for the customers".
"This is a big deal. You don't want this happening the week before Christmas, and especially the week after, when people rely on cellphonesmore.
"It sounds like a major problem."
In cases like these, he said, there was no offer of compensation, which would be a "huge cost", and probably passed on to the customer through higher charges.
He said that the fault "raises concerns about the robustness of systems".
"Everyone's been tolerant of teething problems but sometimes it reeks of the system being too lean.
"It's a bit of a worry."
A Telecom spokeswoman said she could not say how many customers were affected by the breakdown, but those on the older CDMA mobile network were not affected.
The alarm was raised about 7am, when Telecom posted a message on the Twitter social networking website: "We are having some issues with XT sites south of Taupo. Technical teams are working on restoring service."
More than an hour later, a new message went up: "XT sites south of Taupo have no coverage, currently a Severity 1 Fault being worked on to get service restored asap."
About 1.30pm, the company issued another bulletin.
"XT Fault Update: Some sites are back up, this is not yet stable. Please bear with us, work is ongoing to get full XT service back to you."
It is not the first problem with the XT system.
Late last month, the network was down intermittently on sites south of Taupo for a day.
A technical fault caused a loss of mobile service, also including voice, text messaging and mobile broadband, for XT customers south of Taupo.
Telecom started the XT mobile network in May.