Orange will offer mobile phones that can make VoIP calls over a home Wi-Fi router from Monday.

The new service will only be available to Orange customers who subscribe to both its GSM mobile phone network and its DSL broadband Internet access service however. A new phone costing from €99 will mean unlimited calls from home, connecting via WiFi to their Livebox DSL home router. When outdoors, calls will be carried over Orange's GSM network at the usual rates.

Orange first announced its plans for the service, called Unik (for "unique"), on 31 May, the day after its rival Neuf Cegetel announced a similar service called Twin.

Mobile phone networks determine who to bill for calls by accessing the SIM in the mobile phone. To charge for calls routed over WiFi, Unik and Twin use a technology called Unlicensed Mobile Access to access the phone's SIM and authenticate the identity of the subscriber against a database held by the mobile phone network. Once the subscriber is identified, a VoIP call is established over the WiFi and DSL connections and back to the public telephone network.

The Livebox can carry up to three simultaneous WiFi calls, Orange said. Although Orange also offers GSM and DSL services in the UK, Unik may only be available in France. Orange has yet to clarify the situation.

Similar services combining GSM and WiFi are available elsewhere in Europe, including BT's Fusion, launched in the UK last year, Deutsche Telekom's T-One, which opened in Germany in August, and TeliaSonera's service Home Free, now available in Denmark.

While Orange's service will require customers to subscribe to its own broadband and mobile services, other services are more open. Neuf Cegetel's offering is tied only to the company's broadband services: Customers can use any mobile network, while in Denmark, the Home Free service uses TeliaSonera's mobile network but will work over other operators' broadband connections.