Know what your tariff actually covers

Before you dial an international number, it pays to understand the difference between roaming and calling abroad. Roaming is using your UK SIM while you are overseas. International calling is phoning a number outside the UK, even if you are standing in your own kitchen. Many UK plans include roaming in the EU and EEA under fair-use rules, but that does not always mean calls to every European number are included. A call from France to a French mobile might be covered, whilst a call from France to a UK number could be treated as an international call. Check your provider’s zone list, fair-use limit and per-minute rates. If you call overseas numbers regularly, an international add-on or a separate calling plan can be far cheaper than pay-as-you-go rates.

Watch out for premium-rate and unusual number ranges

Some of the most painful charges come from numbers that look ordinary but carry premium rates. In the UK, numbers beginning 09 are premium-rate, and 087, 084 and 070 personal numbers can also be costly. From abroad, satellite services such as +870, +882 and +883 can cost several pounds per minute. Maritime and airline numbers are in the same danger zone. Even freephone numbers abroad may not be free when dialled from a mobile.

• Never call back a missed call from an unknown international number — it may be a wangiri scam, where the caller hangs up hoping you will ring back.
• Check the exact number range before you dial, especially if a directory enquiry service gives you a number.
• Ask your provider to bar premium-rate and satellite calls if you do not need them.
• Remember that calls to some European business numbers may be premium-rated even if the country is in the EU.

Stop voicemail and forwarding from ambushing your bill

Voicemail is a quiet bill-raiser. Retrieving messages while abroad often costs the same as calling your home country, and an incoming call that diverts to voicemail can be charged as an outgoing international call. Call forwarding is another trap: if you forward your UK number to a foreign mobile, you may pay for the forwarded leg. Before you travel, turn off voicemail or switch to a visual voicemail service that uses data. Check for conditional forwarding rules, such as “forward when busy” or “forward when unanswered”, and disable any that point overseas. Tell regular callers to use a messaging app instead. If you must keep voicemail, ask your provider how retrieval is charged and whether you can access messages over Wi-Fi.

Keep mobile data on a short leash

Data is the biggest source of unexpected charges outside inclusive roaming zones. Even inside the EU and EEA, fair-use policies can throttle or charge you if you exceed a monthly limit. Outside Europe, rates of several pounds per megabyte are still common. Turn off data roaming in your phone settings before you leave, then enable it only when you have bought a suitable add-on. Disable background app refresh, automatic photo and video backup, app updates and email fetch. Download offline maps, music and translation files while on Wi-Fi. Be careful with tethering — some providers exclude it from roaming bundles. If you need more data, buy a local SIM or an eSIM for the country you are visiting, and use your UK SIM only for urgent calls.

Set spend caps, alerts and bars

Your provider should offer a spend cap, but the default may be higher than you expect. Log into your account before you travel and set a cap you are comfortable with. Ask for alerts at 50%, 80% and 100% of that cap. Be aware that premium-rate, satellite and some international calls may sit outside the cap, so ask for a separate bar on those services. Check that your cap covers data, calls and texts, and that it applies while roaming. If you are travelling with a child or a work phone, set a lower cap and disable in-app purchases. Keep a note of your provider’s international customer service number, and save it offline.

Run a five-minute pre-trip check

Before every trip, run through a simple routine. Confirm your roaming zone and whether your plan includes calls to the countries you will dial. Buy any add-on in advance. Turn off voicemail and overseas forwarding. Disable data roaming until you know the rates. Download offline maps and entertainment. Save local emergency numbers and your provider’s contact details. When you land, send a message rather than making a test call, and check your usage after the first day. If a charge looks wrong, query it quickly with your provider — many will investigate unexpected premium or roaming charges if you raise them promptly. A few minutes of preparation is usually enough to keep your bill predictable.