Customers should be wary of charges levied when they use their phones abroad - a business that generates up to 15 per cent of mobile firms' revenues, writes Laura Slattery

You've fastened your seatbelt, made sure everything is in the upright position and ready for take off when a polyphonic ringtone crescendo emits from your carefully stowed carry-on luggage.

Cabin crew will instantly remind you that for the duration of the flight you must keep your mobile phone powered off.

What they won't add is that anyone who wants to avoid the possibility of sky-high bills should probably keep it that way until their flight - their return flight - has safely touched down.

Roaming rates - the charges mobile firms levy on customers when they make or receive calls abroad - are a complex business, one that is estimated to raise up to 15 per cent of operators' revenues.

Vodafone customers who suffer separation anxiety from their handsets on holiday will be pleased to hear that the Republic's largest mobile operator has introduced a new flat-rate roaming service that makes the charges a good deal simpler.

Under Vodafone World, different roaming tariffs apply to just five zones worldwide. In each zone, there is one set of rates for roaming with a local Vodafone or partner network and another, more expensive, set of rates for those who dare to hop onto a rival network.

To guarantee that the customer knows which network is the Vodafone one, they will automatically receive a text message (SMS) to inform them of the local Vodafone or partner network on arrival. They can then manually select this network on their handset.

Failure to do so will result in higher call charges: for example, €1.59 per minute in the UK compared to the Vodafone network charge of 59c.

Vodafone World replaces the old system of about 4,000 separate rates with different charges for each country and network, and peak/off-peak rates determined by the local operator.

Just to make it even more confusing for travellers, in the past it was also sometimes cheaper for both Vodafone and O2 mobile customers to abandon those companies while abroad and roam on other networks.

Vodafone says its new roaming service offers 10-40 per cent reductions for key holiday destinations, because customers can now take advantage of Vodafone's "global footprint" and avoid the charges levied by its competitors.

But in some countries, however, the cost of roaming has gone up.

The cost per minute for a post-paid (contract) Vodafone customer of receiving a call in Australia used to be either €1.31 (off-peak) or €1.77 (peak) on Vodafone's Australian network. It is now €1.99 at all times.

The cost of making calls to Ireland from Australia has also gone up by a small margin.

In fact, Vodafone's prices down under compare unfavourably with those enjoyed by O2 and Meteor customers.

But O2 customers will only get the 37 cent per minute price for calls shown in the table opposite if they roam on the most competitive Australian operator, Optus.

If their mobiles wander onto the Vodafone network, the cost leaps to €1.30 per minute.

O2 customers can also roam for less than Vodafone subscribers in the US as long as they use the Cingular or T-Mobile networks. If they receive a call from home while they're on the AT&T Wireless network, it will add €1.59 to their bill for every minute spent on the phone.

Things get a little more straightforward for O2 customers closer to home.

In selected European countries, the cost of making and receiving calls for bill-paying customers is 59 cents on O2 networks, 79 cents on "preferred" networks and 99 cents on other networks.

O2 won't automatically send an SMS to customers telling them they can get a better rate, say, on "preferred" network Radio Mobile than they can on Cesky or Eurotel when they're on a city break in Prague. But a list of the preferred networks is available on its website.

So what about pre-paid customers? First of all, their ability to roam is curtailed. In north America, it is impossible for pre-paid customers to use their mobiles, while O2 also says its pre-paid customers cannot roam in Australia.

It may come as little surprise that roaming, like domestic calls, frequently costs more per minute for bill-free top-up buyers than it does for contract customers.

For example, a Vodafone Ready to Go customer in Spain will pay 99 cents per minute to receive a call, compared to 69 cents if they have a bill.

In popular European holiday spots, O2 charges pre-paid Speakeasy customers 99 cents for all calls. Text messages cost 39 cents to send - cheaper than Vodafone's text rate (but not Meteor's).

Some 70 per cent of its customers are pre-paid, according to a spokeswoman. "When they go on holidays, we know from their call patterns that the first thing they do when they reach a hotel is send a text back home."

Roaming activity typically peaks during the summer months, as the skies around Irish airports become more congested.

But some mobile users are perpetual roamers, knocked about from network to network if they so much as walk 10 feet down the road.

These people are likely to live in the Border regions of the Republic and Northern Ireland, and they don't have to actually cross the Border to be charged for roaming.

Both Vodafone and O2 admit that signals from UK mobile companies often get entangled with theirs, and vice versa. This can prove costly for locals who find themselves accidentally roaming on UK networks.

Only post-paid customers can do anything about this.

The O2 spokeswoman says its "all-Ireland" flat-rate option is "very popular" with these accidental roamers, as it means they won't be charged for receiving calls when travelling in Northern Ireland and will incur domestic rates when calling cross-Border.

The option can be selected as the one free add-on included in O2's Choices price plans. Otherwise, for customers who want free text messages or cheaper weekend rates as their free add-on, the option costs €7.50 per month.

Vodafone's Ireland-Wide tariff, meanwhile, costs €5 per month, a fee that has been criticised by the Minister for Communications, Mr Ahern.

The Minister believes that all operators should offer all-island flat rates with no sign-up charges and that they should also be available to pre-pay customers.

The same danger of accidental roaming exists overseas, and now that Vodafone is penalising customers for making or receiving calls on rival networks, it is even more important for holidaymakers to be vigilant.

Once Vodafone customers have manually selected the local Vodafone or partner network, they should stay on it in the majority of cases, says Mr Daragh Kelly, Vodafone's roaming marketing manager.

But not always.

"Basically it is the nature of the technology that if you can't get coverage on the network, it will look for the next best coverage. So if you're in Spain and you're on the Vodafone network, for some reason, if you get on a bus, you might get kicked off the network and onto Telefónica."

At which point, the cost of making a call back home will jump from 89 cents to €1.19 per minute.

Despite Vodafone's clearer charging system, most consumers are still likely to use their phones only sparingly when overseas and try to decipher their bills later. With some international roaming charges exceeding 15 times the cost per minute for domestic calls, some may wonder why consumers bother to pack their mobile at all.