What is international roaming?

Phoning home can be an expensive business

'International roaming' is the issue at the centre of the EU probe into the European mobile industry.

It might sound tremendous fun - but it is, in fact, just telecom industry jargon for making calls on your mobile phone when you are abroad.

The mobile phone may be a cheap convenience in the UK, but using it overseas invariably turns it into an expensive luxury.

All the UK mobile phone companies have 'roaming' agreements with foreign operators that allow customers to use their phones when travelling - but the costs are nearly always very high.

Not only is it expensive for you to call out but it is also expensive for other people to call you.

Hard to calculate

The owner of the phone tends to get charged for the portion of the call beyond the UK.

This can be true even when the operator you are using is a subsidiary or sister company of your home mobile phone company.

As a result, international mobile calls can cost up to £3 a minute.

But, critics say, there is no clear reason why the charges are so high.

And constant protests from consumers have led to an EU competition probe - and this week's raids on mobile companies in the UK and Germany.

Roaming calls are hard to calculate because of the number of factors that have to be taken into account.

These include: the network operator you are using to make the international call, the time of day, the contract agreement and the roaming agreement your operator has with the foreign phone company.

Some companies have launched international calling cards that give subscribers a discount.

But these calls are still more expensive than using a landline.