Mobile operator Vodafone could face legal action over its efforts to cripple VoIP on mobile phones.

"We believe Vodafone's stance is not lawful, and we are considering our position," said James Tagg chief executive of Truphone. Vodafone is failing to meet interconnection obligations, and blocking competing websites, he said, as well as disabling Internet telephony on handsets.

Vodafone is refusing to connect calls to Truphone's range of mobile numbers, Tagg says, in breach of telecoms regulation. Truphone has a range of numbers (beginning 07978 8XXXXX) with which it can function as an actual mobile operator - so that Truphone numbers appear in a phone's call log, and can receive SMS messages. Unfortunately, Vodafone customers will simply find that calls to those numbers are blocked, whether they are made over WiFi or the Vodafone network, says Tagg.

The operator is also blocking VoIP websites including Skype, to users of its mobile Internet service, says Tagg - a move which would be against EU competition regulations, he says: "European telecoms legislation, gives operators an obligation to interconnect, and to offer unfettered access to services."

Vodafone-provided N95s include SIP and Internet telephony, but the Internet telephony is locked so it can't be used, a move which could be illegal, even on subsidised handsets, says Tagg, even though it doesn't prevent users from downloading other VoIP applications which include their own telephony features.

“Customers can download VoIP applications if they choose to do so or can use VoIP services via a laptop and data card,” said a prepared Vodafone statement, and other VoIP providers bear this out: "Our service works on Vodafone and Orange," said Gerry O'Prey, chief executive of WiFiMobile, a VoIP company specialising in connecting dual mode phones to business PBXs.

While operators have argued that they have a right to determine the software on subsidised handsets, Tagg claims that the equipment provided by the operator is independent of the contract. Users expecting integrated Internet telephony on an N95 would have grounds under consumer law for sending it back, but Vodafone's duties go beyond that, he says.

In more than half the world, operator subsidy is illegal, said Tagg. So - for instance -Vodafone has promised not to disable Internet telephony on N95s it sells in Australia. Where subsidy is legal, it is restricted, he says: "The operator has a monopoly on the line into your pocket," so it is not allowed to lock down services and create walled gardens. "There is lots and lots of case law that has been fought out on the PC," he said referring to Microsoft's desktop monopoly.

While VoIP services can be downloaded to a crippled N95, they won't be "properly integrated", says Tagg. Using an alternative SIP stack could mean shorter battery life, compared with the Nokia SIP stack: "They've spent two years and millions of pounds ensuring it's integrated into the phone, and optimised for battery life," he said. Integration also allows features like calling a VoIP number back from the phone's call log - which some analysts reckon is how fifty percent of calls are made.

The complaints go beyond those made last month, that Vodafone and Orange have disabled Internet phone features on the top-end Nokia N95 handsets, making it difficult for users to make cheap calls at Wi-Fi hotspots.

Vodafone has not yet given a response to Truphone's claims that it is blocking interconnection to Truphone numbers. Instead, the operator sent us a statement it made last week (and we referred to it here), on the Nokia N95 issue, arguing that it has disabled Internet telephony for the consumer's protection.

"Vodafone believes that VoIP-over-mobile is not yet a mature service proposition as it does not have guaranteed quality of service, and would fall short of the customer experience demanded of any service we launch," the statement says. "To ensure a solid end-to-end customer experience, this service would require in-depth testing, billing integration and customer service support which is currently not available."

"There is also a misleading perception that VoIP services are "free". This is not the case when it comes to using VoIP over mobile where customers will need to use data connectivity to establish a service. By doing this, there is a risk that customers could incur unnecessary charges when competitive mobile tariffs are likely to be a more cost-effective choice."