Questions have been raised about whether Telecom's 0867 numbering scheme would stand up in court.
IT lawyer Craig Horrocks believes Telecom must prove users are making data calls rather than voice to be able to charge 2 cents a minute.
Telecom plans to charge 2 cents a minute for ISP users who don't dial its new 0867 numbers.
"If you say: 'I dialled that modem for a voice purpose, you prove I used it for data', then it's not enough for Telecom to say: 'You called an ISP number therefore it's a data call'," says Horrocks, who is a Computer-world contributor.
He believes if Telecom is able to prove you are using the connection for data traffic instead of voice it would be invading your privacy and would be in breach of the Privacy Act.
"At the moment, [Telecom] can tell that you've made a connection with a particular subscriber but they don't know what you're talking about." Horrocks says to prove you are using the call for data and not voice, Telecom will have to know whether you are downloading data.
Telecom spokesman Glen Sowry says Telecom cannot do that.
"We have absolutely no visibility of where a customer goes on the Internet. It would be logistically impossible to monitor whether a call is voice or data."
Telecom decides whether a call is voice or data at the earliest stage: "It's based on the number dialled."
Sowry can't understand why anyone would call a modem and listen to the noise. "We hope common sense would prevail." Horrocks says that isn't enough.
"If they prove you made a data call then they can charge you but they've breached your privacy.
"If they can't prove you've made a data call they can't charge you and you can carry on using the old dial up number for free."
"If [customers] didn't have to pay the two cents per minute, we wouldn't be doing anything at all," says Hamilton-based ISP The Wave's managing director, Wayne Attwell. He would rather stay on the existing payment structure and says the only reason he's shifting to 0867 is because of the surcharge.
A Clear communications spokesman Ross Inglis says: "It's a question that hasn't been adequately answered and Internet users are entitled to an answer."
"That's a very interesting suggestion and we'll be very interested to see how it progresses," says Internet Society executive director, Sue Leader.
The Privacy Commissioner says he does not investigate hypothetical situations.