2degrees has followed Vodafone in switching on "HD Voice", a technology that improves the quality of mobile calls.
However, it has emerged that the advance will not yet benefit people when they are calling between mobile networks.
Vodafone enabled HD Voice on its network in October, with customers likening it to switching from AM radio to CD-quality sound.
Telecom has said it is investigating HD Voice but has so far been noncommittal.
2degrees said the difference in call quality had to be "heard to be believed".
However, for the technology to deliver benefits, both people on a mobile call must be making an "on-network" call, for example a call from one 2degrees mobile to another, it said.
They must also both have a handset that supports the technology, such as an iPhone 5, HTC One or Samsung Galaxy S4.
Spokeswoman Charlene White said "a great deal of work" would need to be done before it was possible to enjoy HD Voice on calls between 2degrees and Vodafone.
"It is something we'd like to see happen."
- The Dominion Post