Suppliers of Voice over IP (VoIP) services will have to provide clearer information, especially about continuity in the event of power failure, under new rules from telecoms regulator Ofcom.

A survey of more than 250 UK businesses published earlier this month found that 80 percent expected IP telephony to help transform their working practices over the next two years, while Ofcom predicts that up to 3 million consumers will be using VoIP services by the end of the year.

Under the new rules, VoIP providers will have to state clearly whether their service will work in the event of a power failure, Ofcom said.

Many VoIP services depend on a powered router for internet access and will not work if power supplies are cut off, although many VoIP gateways include "lifeline" support falling back to the landline when poer is interrupted (see this and other reviews), and telecoms firms such as BT offer services where both VoIP and regular landline calls can be made from the same phone, allowing call sto be made during a power outage.

Service providers will also have to be clear about whether or not they allow users to make calls to emergency services, Ofcom said. They must also disclose whether or not they provide directory enquiry and operator services, and whether users can ask for an itemised list of calls.

Providers that do not offer emergency service calls and whose services do not work without a power supply must display warnings on their products, Ofcom said.

VoIP firms will also have to disclose whether customers can keep their same phone number if they switch providers.

The regulations - the first for the VoIP industry in the UK - take effect in June. They were drawn up following a public consultation last year.

Ofcom will hold a further consultation this year to determine whether VoIP providers will be required to offer access to emergency services, since being unable to reach blue light emergency services could be detrimental to users, it said.

US regulators have banned companies from advertising VoIP services if they do not allow people to call emergency services.

"In assessing the need for any new requirement, we will continue to carefully consider the impact of such regulation on market entry, innovation and competition," Ofcom said.