Mobile VoIP provider Truphone is extending its service to give users free or cheap calls over the cellular network, when they are away from Wi-Fi hotspots. Meanwhile, a rival consumer service has launched with backing from UK retailer Tesco.

Truphone Anywhere dials a gateway on a local number, which then connects through to the destination number, saving money if it is an international call. Unlike some other services, this is transparent, with the call set-up handled automatically after the user dials the remote number. It is enabled partly by a recent Truphone acquisition, SIM4Travel, which provides cheap international calling through gateways in Europe.

"We've put together our mobile numbers and interconnect, with SIM4Travel's home location register (HLR)," said James Tagg, chief executive of Truphone. The company has its own number range, which has enabled it to act as a mobile operator, and win against operators including T-Mobile, which challenged this. In April, it raised another £16.5 million in venture funding to add to £12.5 million which it raised in 2007.

The idea of mobile VoIP is gaining ground in the UK, with Australian operator FreshTel launching a trial backed by a Tesco, and with a "buy one get one free" offer. FreshTel uses a VoIP client similar to Truphone's installing on Nokia E and N series phones, and operates on any Wi-Fi.

New FreshTel subscribers get £2 credit for a text which costs £1 in a limited beta-test deal, launched at the Wireless 08 show in London. The text starts an installation process which - in FreshTel's demo to Techworld, at Wireless 08 - was quick and painless, needing no web registration. The company claimed benefits compared with Truphone, including the use of the subscriber's regular mobile number.

"We're using other brands, to access their customer base," said Rhonda O'Donnell, chief executive of Freshtel, explaining the tie-up with Tesco, which owns around ten percent of the company.

Truphone's Tagg said that FreshTel was behind Truphone, in that it does not save money on incoming regular calls, and does not work away from Wi-Fi.

Both services are likely to go for a higher profile - with FreshTel hoping for more direct promotion by Tesco after this trial, and Truphone hovering on the verge of services that could put it on a more even footing with regular operators - for instance, perhaps, offering its own SIM.