Telecom switched off one of its networks overnight, rendering the mobile phones it serviced near-useless.
In a move flagged over a year ago, the company shut down its 11-year-old CDMA network at midnight and is now operating only its XT network.
The two networks are based on different technology, with XT being seen as being more similar to Vodafone and 2degrees' infrastructure.
CDMA customers are now not able to make or receive calls, send text messages or use mobile data.
CDMA phones can be safely disposed of and recycled, with Telecom offering collection services at its stores.
Acting chief executive Chris Quin said the company has been pleased with the uptake on XT and believed there was a high awareness that the CDMA network was closing.
The company won't reveal how many of its customers were still on the old network at the time of the shutdown.
A spokesman said yesterday the company stopped selling CDMA phones at Christmas 2009. It was prepared to refund any phone credit pre-paid customers had left on their phones.
XT was launched in May 2009 with an advertising campaign featuring Top Gear star Richard Hammond but was dogged by outages six months later which saw the company stump up $5 million in compensation to customers.