Telecom customers have until the end of Tuesday to make sure their mobile phones are updated before the company pulls the plug on its outdated CDMA network.
From midnight, customers using the network will no longer be able to make or receive calls, send texts or use mobile data.
The CDMA network is being replaced by Telecom's XT system, which was introduced in 2009.
The XT network was initially plagued by faults, the most serious of which prompted Telecom to announce a $10 million compensation package.
Telecom will not say how many of its customers remain on the CDMA network. However, acting chief executive officer Chris Quin says the company has been pleased with the uptake on its XT network.
Telecom says after the closure, CDMA customers may still see coverage bars on their phones while the network is powered down over the next few weeks.
Telecommunications Users Association chief executivePaul Brislen says he suspects some people are still using CDMA phones, and they might see the shutting down of the network as a good time to switch providers.
"2degrees is doing a fairly loud marketing campaign at the moment devoted to getting people over to 2degrees instead of moving to XT."
Mr Brislen says he expects there will be quite a lot of migration between providers in the next few weeks.