A major global roaming network glitch is affecting Telecom and 2degrees mobile customers.
The fault means affected customers overseas can't use their phones properly.
Customers have been sporadically unable to make calls, texts or access data since yesterday morning.
On its website, Telecom said it was trying to solve the problem.
"Our apologies, our international roaming service is experiencing difficulties worldwide," it said.
"Roaming customers may be unable to make or receive calls, send or receive text messages or data."
2degrees has also posted similar messages.
Users of the services have taken to Twitter with their concerns.
"My partner is in oz at the moment and his phone has stopped roaming - his workmates too. will this be fixed soon? Pls :-)," wrote Twitter user Anjuli.
An automated message on Telecom's service phone number says it is giving the problem "the highest of priorities".
