It had to be too good to last. A few thousand Internet users have been surfing away in the evening and at weekends without paying a bean for their phone calls, thanks to their local cable companies. Now the companies have cottoned on: some are withdrawing free calls, while others are thinking about it.

Britain is the only country where television cable companies are allowed to provide telephone lines. They have been building up this business by aggressive pricing, and three of them - Videotron, Nynex Cablecomms and Diamond Cable Telecommunications - have offered free off-peak calls in an effort to cut into BT's monopoly.

The phones being connected must both belong to a cable company customer, which is why several Internet service providers have linked themselves up to the companies. That means Pavilion Internet subscribers in Brighton, for example, can log on to the World Wide Web in the evening for as long as they like without paying for the phone call.

Pavilion is linked to Nynex. So are Mistral Internet in Brighton, Aladdin in Portsmouth and the Direct Connection in Warrington. Videotron phone lines are connected to ExNet Systems and the Direct Connection in London while Aladdin has Videotron lines in London and Southampton. The Internet (a company) in Nottingham uses Diamond phone lines.

Videotron and Diamond customers need pay only pounds 7 a month for a telephone line: they do not have to take the television service as well. Nynex users must rent the phone line, the standard TV service and a premium movie or sports channel to qualify; a total of almost pounds 30 a month.

But, as we reported two weeks ago, U-Net in Manchester received a letter from Nynex saying it intended to withdraw its free local call offer from Internet service providers' Nynex lines, claiming that its network is ill-equipped to deal with users hogging its exchange switches. This week a spokesman insisted that Nynex had made no firm plans and was still investigating all the possibilities for change. He said the letter to U-Net was premature, but refused to comment on suggestions that Nynex was brushing aside the competition in order to set up its own Internet provider.

Diamond Telecommunications has also revealed that it is planning to stop free calls to Internet Service Providers with Diamond telephone lines. A spokesman said that its free calls scheme was never intended for business services. Diamond is currently running a pilot Internet service offering digital ISDN and standard modem access, and the full service should be up and running by October. It will not however, give its subscribers free access.

Videotron, the London and Southampton cable franchise, with 1.3 million customers, says it has no intention of giving up free calls to Internet providers because it profits from the voice calls Internet users make after going online.

But there are warning clouds ahead. Videotron's contract states that only voice calls are eligible for the free phone call scheme. At the moment the company is choosing to ignore this, but it is looking at the options for setting up an Internet Service Provider of its own. If that happens, it is possible that Videotron would withdraw free phone calls to other service providers.