Around 8000 people will wake up on Thursday morning with defunct phones.
Mobile company 2degrees has switched off its 2G network overnight.
Spokesman Matt Bolland says they'll be reusing the airwaves to make things faster for the 4G network.
"The phones no longer work, but your number and your credit still will be available so we can get you over to a new handset pretty quickly.
"But it's important that you do make the change because the phones don't work, even for emergency calling."
Mr Bolland says the old network was "wasn't being used by a lot of customers".
"All it did really was voice and text, whereas 4G, the latest version, is all about data - doing lots of things like video, using apps."
Customers still on the 2G network can head down to their nearest store to get a new handset.