Nokia has begun testing technology that lets users make calls on both cellular and WiFi networks and switch between them without interruption.

Nokia's new 6136 phone automatically transfers voice or data connections from GSM networks to WiFi when the device recognises a compatible wireless network, and vice versa, said a Nokia spokesman.

Around 50 people in Oulu, Finland, are testing the new service as part of a two-month pilot in co-operation with Finnet and the City of Oulu. The city has been offering free access to WiFi hot spots since last year as part of a project to spur wireless Internet service.

The Finnish vendor is one of several vendors, including Motorola, to offer handsets with UMA technology, which enables a "seamless" handover of voice and data connections between GSM mobile and WiFi or other local wireless networks.

BT launched one of the world's first UMA commercial services last year using Bluetooth technology, and plans to extend its service in the third quarter of this year to WiFi networks with Motorola's A910 UMA-enabled phone.

TeliaSonera in Sweden and Finland and T-Mobile in the US plan UMA services in the coming months.

UMA allows consumers to enjoy better indoor coverage and lower calling charges when within range of a compatible WiFi terminal, by having it carry their calls over the fixed broadband network using VoIP.

UMA handsets such as Nokia's 6136 create a secure tunnel to the network operator's UMA Network Controller, which uses the handset's SIM (subscriber identity module) to authenticate the caller with the operator's billing system just as it would over a cellular network.

The new Nokia 6136 phone, which Nokia announced in February, will be available in the third quarter, according to Dawson. The phone, costs €275 and comes with a 1.3 megapixel camera and removable micro SD memory card.