BT is getting back into the mobile phone business with the launch of Mobile Sense, an online service that allows users to swap tariffs as often as they like.

In the past mobile operators have tied customers into a specific tariff for at least a year, but BT says that with Mobile Sense customers can simply log on and swap their tariff for one that suits them better.

To sign up for the service customers must visit the website, and choose a price plan that best suits the way they use their mobile. For example you are asked how much time you spend on your mobile and during what hours.

The cheapest plan on offer is £15, off-peak. This provides you with 200 minutes of calls to land lines, 25 calls to mobile phones but no text messages. The monthly line rental, included in the cost is £10.49. Costs beyond your free minutes are 30p peak or 5p off-peak to landlines, voicemail and O2 phones or 45p peak and 35p off peak to other mobile networks. Text messages are 12p. All the prices are based on O2, ex-BT Cellnet, tariffs.

T-Mobile's closest equivalent is Free Time 750, which costs £13.99 and provides you with 750 free off-peak minutes to landlines and T-Mobile phones, and 50 free text messages. This looks like a better deal, but pricing beyond your free minutes is slightly higher at 40p peak to landlines and T-Mobile phones and 50p to other networks, although off-peak prices are less at 5p to landlines and T-Mobile numbers and 30p to other networks. Another bonus is free access to voicemail.

If you are a light user and already have a handset, you can make even more savings by going for a contract-free deal with Virgin Mobile, where you only pay for calls made and texts sent, which can work out a lot cheaper than a contract-based deal. It charges 15p per minute for the first 15 minutes of calls everyday, and 5p per minute after that for calls to other Virgin Mobile customers and national UK numbers. Text messages are 10p per message, voicemail is free, and calls to other networks are 35p per minute. But because you don't pay for a contract, handset prices are high, although porting your SIM card only costs £10.

But BT's main selling point isn't the cost of its price plans, but the fact that you can swap tariffs as often as you like. However this isn't such a revolutionary tactic, as many operators, including T-Mobile, will now allow customers to change tariffs after the first month of their contract is up.