T-Mobile cannot advertise Apple's iPhone in Germany because of an injunction that disputes its claims of allowing "unlimited" internet usage on the device. (Ed Ou/Associated Press)

A German court has sided with wireless carrier T-Mobile and banned the use of Voice over Internet Protocol on Apple Inc.'s iPhone.

The Higher Regional Court of Hamburg issued an injunction on Thursday preventing the download of an application called "Sipgate," which allows iPhone owners to place calls over the device's internet connection rather than over T-Mobile's cellphone network. Users could then make free calls from wherever there is a Wi-Fi connection, saving themselves airtime, which they must pay T-Mobile for.

The court said the application, which was in beta test phase, would lead users to "jailbreak" their iPhones.

Sipgate's Thilo Salmon told the VoIP and Gadgets blog the company was not allowed to argue its case before the Hamburg court prior to the injunction being issued, and that T-Mobile had spent two months trying to find a court that would side with it. The injunction prevents Sipgate from advertising and distributing the software.

Sipgate did win a victory however, as the same court upheld an injunction against T-Mobile issued in July that prevents the carrier from advertising the iPhone in Germany. Sipgate said T-Mobile's advertising, which claims "unlimited" internet usage but with a host of restrictions — including a ban on VoIP — was misleading, a position the court agreed with.

"I guess that at least eases the pain," Salmon said.

U.S. legal authorities have also clamped down on carriers that have claimed "unlimited" usage. As part of a settlement with the New York attorney general last October, Verizon Wireless was forced to repay $1 million US to cellphone customers it had cut off for "abusing" their "unlimited" mobile internet service.

"This settlement sends a message to companies large and small answering the growing consumer demand for wireless services. When consumers are promised an unlimited service, they do not expect the promise to be broken by hidden limitations," said attorney general Andrew Cuomo. "Consumers must be treated fairly and honestly. Delivering a product is simply not enough – the promises must be delivered as well."

Canadian carriers — including Telus Corp. in August — have come under fire from customers for also offering "unlimited" services that are limited in many ways, but regulators and legal authorities have taken no action.