Mobile operator 3 UK will allow users to enjoy unlimited Skype-to-Skype calls and messages without ever having to pay, as long as they have a compatible handset and a SIM card from the operator, it said on Thursday.

The new pricing will be rolled out in two phases; from May 1 there will be no data charges or top-up fees for either contract or pay-as-you-go customers who use Skype on 3's U.K. network and one of its phones, according to a statement.

Soon thereafter the offer will expand to anyone with a compatible unlocked 3G handset in the U.K., the statement said.

3 UK offers a range of Skype compatible phones, including models from Samsung, Nokia, Sony Ericsson and LG. At its web page (Three.co.uk) users can check if their phone works with Skype.

Skype is nothing new to 3 UK. It already offers mobile phones that come with tight Skype integration, and subscribers use 1.5 million minutes of Skype calls every day, according to the operator.

The Skype push is all about 3 UK trying to gain market share, according to Mark Newman, chief research officer at market research company Informa Telecoms and Media.

The U.K. is an intensively competitive market. There are five operators and 3 is in fifth place, according to Newman. It needs a way to increase its market share, and hopes that the publicity this brings will help it gain new customers, he said.

Not all operators are as fond of Skype as 3 UK. Recently, T-Mobile in Germany made headlines when it said it would block the use of Skype on the iPhone.

"Skype wants to be every mobile operator's best friend, but its too early to say how successful it will be. Voice is still mobile operators' core business, and it is still 80 percent of their revenues," said Newman.

However, operators have more to gain by working with Skype than working against it, according to 3 UK.

Subscribers that use Skype are, for example, less likely to switch to another operator, and they also use more traditional voice minutes and send more messages by SMS, it said.