"Roaming", as using your mobile on a foreign network is known, is possible for phone users connected to all four UK mobile networks - Vodafone, Cellnet, Orange and One2One.
There is no need to change your number and no need for callers to dial an international prefix to reach you. This is certainly convenient but it is also expensive. Within the UK, calling a mobile costs more (as much as 37.5p a minute) than calling a normal phone. But if this mobile is taken abroad the cost soars for the mobile owner.
The caller still pays the mobile rate, which is usually more than the cost to call a fixed phone in Europe or the US, but the subscriber also pays to listen to them. Cellnet and Vodafone charge 55p per minute, plus VAT, for calls received in France or Germany. Charges increase dramatically outside Europe: Vodafone's partner network in Hong Kong, for example, charges pounds 1.26 for incoming calls from the United Kingdom.
At these prices a few calls on holiday can mean a large bill when you come back. On most mobile phone price plans, subscribers have to request roaming capability, but once the service is there it is easy to forget the true cost. Paying several pounds to receive an important call might be worth it in an emergency. But it could well be a painful way to speak to a salesman, wrong number or even an absent friend.
The networks justify these charges by saying that callers have no way of knowing where in the world a mobile user is. They argue that it would be unfair to saddle callers with an international charge for ringing, say, Bali, when they thought the phone they dialled was in Bracknell, Berkshire.
"They might not want to call you in Australia, or even know you are there," says Libby Pritchard, for Vodafone. "Suddenly expecting callers to pick up a call to Australia is unreasonable." Vodafone thinks travellers will understand that they have to pay part of the cost. "We believe the flexibility is worth it. You can be called on your own number, and can make calls wherever you are."
Of course, if it is your family making the calls but your employer picking up the tab you may be perfectly happy with this set-up. But some people will end up paying both bits of the bill.
The combined cost to both parties for a one-minute call from the UK to a GSM mobile on Vodafone, roaming in France or Germany, comes to pounds 1.02 a minute including VAT. By comparison BT's charge, even at peak rate, from a domestic phone to another standard phone abroad, would be 28p.
Furthermore, some of the most expensive networks for roaming calls are the cheapest in the UK. One2One generally costs less to use than Cellnet or Vodafone in the UK. But receiving a call in France on Cellnet or Vodafone GSM costs 55p plus VAT per minute, while roaming on One2One's partner network in France costs 89p a minute (including VAT).
Mobile operators point out that users can switch off their phones and collect messages from their answering services every day. They also say their phones are better value for calling back to the UK than using a hotel phone.