Including free roaming in trade deal negotiations could save UK holidaymakers significant amounts of money.
There's been no extra costs for using your phone in European Union countries since June 2017.
But Which? research reveals holidaymakers still face punitive roaming charges in the USA, Australia, New Zealand and Japan - all countries the UK is in negotiations over trade deals with - and in the case of Japan, has agreed in principle.
In the USA, Australia or New Zealand, Plusnet customers would also face a £30 fee to upload a 5MB photo, while Tesco Mobile and Virgin Mobile customers would have to pay a steep £25.
Plusnet, Tesco and Virgin and BT customers could also face a high bill for visiting Japan. Uploading a 5MB photo would cost Plusnet customers £30 and customers of the other three networks £25.
Here we reveal the most and least expensive networks for each country, why trade deals should include free roaming, and how you can save money today.
If no trade deal is agreed between the UK and EU, networks will be allowed to charge you extra for using your phone in the EU from 1 January 2021.
But that doesn't mean they will. EE, O2, Vodafone and Three have said they have no plans to charge extra. Which? wants the existing ban on roaming charges to be maintained, to ensure consumers are not faced with higher charges for calls and data when travelling.
Free roaming works in the EU because there is a limit on the wholesale rates that networks can charge each other, as well as a ban on retail surcharges.
Roaming charges vary hugely between networks and even between customers on the same network, if they've bought a specific travel package (more on those below).
To simplify matters, we've looked at charges in different countries for using your phone in four typical ways:
Bear in mind that even if you don't use your phone, it may still be using data in the background, such as automatically backing up photos in the cloud.
Your network may cap your monthly bill, and it's possible to set your own cap - avoiding exorbitant charges, but leaving you cut off if you don't have wifi. You can find out more about caps below.
It's possible to roam for free in all three countries - providing you're a customer of Three.
The US, Australia and New Zealand are among the 71 countries in Three's 'Go Roam' scheme, which lets you use your UK call, text and data allowance for no extra cost.
At the other end of the scale, Plusnet customers face the highest prices for data.
Back in the UK, the prices charged by the two networks are similar, with Three Sim-only plans starting from £5/month for 1GB data and unlimited calls and texts, whilst Plusnet starts from £6/month for 2GB data plus unlimited calls and texts.
Customers of BT Mobile, EE, O2, Sky Mobile and Vodafone currently have to pay daily for travel data passes covering data use in the US, Australia and New Zealand. As these are flat fees (£6, £4.80, £4.99, £6 and £6 respectively) they tend to benefit heavier data users, whilst light users will get charged for any use.
Here are the most and least expensive networks if you're headed stateside or downunder:
Travellers to Asia could save even more if free roaming was included in trade deals.
Neither country is in Three's Go Roam scheme, although Three was the cheapest network for a call home (£2 for three minutes).
The cheapest network for data when travelling in either country was Giff Gaff, charging 58p to stream a four minute song from Spotify, or £1 to upload a 5MB photo in Japan or India.
Japan or India-bound travellers are more at risk of a shock bill when they get home due to the huge disparity in prices for uploading a picture, as our graph indicates:
All mobile network providers have a 50 euro (approx. £45) monthly cap on roaming under current EU rules to prevent customers from racking up expensive bills.
You can also set your own cap, or choose to remove your cap altogether.
Some networks will warn you if you're approaching your limit. BT, for instance, sends you a text when you reach 80% of your cap, as does Plusnet.
|Mobile provider||Monthly data cap|
*These network providers offer customers a travel pass or daily cap ranging from between £4.80 to £6 a day.
**ID Mobile customers can choose their own monthly data cap and is set at £5 automatically if no cap is selected at the point of sale.
You could just turn off roaming - even if you don't use the internet, or call home, your phone may still be using data in the background (this can also be turned off, for some apps). You'll still be able to use wifi, where it's available.
Long term, you could switch networks, to make use of Three's Go Roam scheme, or the travel passes offered by several networks if you'll make full use of them.
It is possible to buy local Sim cards in some countries when you travel, although your phone will need to be unlocked, and you could pay extra fees to call the UK.