Start with how you actually travel

Most people choose a monthly plan by comparing headline prices, then discover six weeks later that the deal works beautifully in one country and badly everywhere else. If you are on the road most months — a few days in the Netherlands, a week in Poland, a long weekend in Spain — the plan you need is not the cheapest one on a comparison site. It is the one that behaves consistently across the countries you actually visit.

Before you compare anything, write down three things: the countries you visit most often, roughly how many gigabytes you get through in a typical month, and whether you need to ring landlines and mobiles abroad rather than just use data. Those three answers will rule out more plans than any price table will.

Roaming allowances and fair use rules

Within the EU, roaming works on a "roam like at home" basis, which means your domestic allowance of data, minutes and texts should follow you across member states. In practice, that promise comes with fair use conditions, and they are where most travellers get caught out.

• Fair use data caps. Many providers cap how much of your allowance you can use abroad each month. It is often expressed as a formula based on what you pay, and it can sit well below your domestic allowance.
• Time-based limits. Some tariffs assume you live in the UK and only roam occasionally. Spend more time abroad than at home over a four-month period and a surcharge may apply.
• Countries outside the EU. Switzerland, Norway, Iceland, Liechtenstein and parts of the Balkans are handled differently. Some plans include them; others charge per megabyte from the moment you land.
• Speed throttling. A plan may advertise "unlimited" data but reduce your speed after a set volume, which is painful if you rely on video calls for work.

Read the small print for the words "fair use" and "roaming allowance" specifically. If a plan's summary does not mention them at all, that is usually a sign the detail is buried rather than absent.

International calling is a separate question

Data is the easy part. Calling is where monthly plans differ most sharply, and where a tempting data deal can turn out to be expensive. There is a real difference between calling a European mobile from your UK plan, calling a landline, and calling a number outside Europe altogether.

• Inclusive international minutes. Some plans bundle a set number of minutes to a list of countries. Check whether mobiles are included, not just landlines.
• Per-minute rates outside the bundle. Once your inclusive minutes run out, rates can jump considerably. Ask what the rate is before you exceed the allowance, not after.
• Receiving calls abroad. Incoming calls can be charged in some non-EU destinations, so a call you did not ask for still costs you.
• VoIP and messaging apps. If most of your contact happens over data-based apps, prioritise a generous data allowance and treat voice minutes as a backup.

It also helps to know which network your calls actually route through. A plan that resells capacity on a partner network may offer excellent coverage in one country and patchy service in another, even though both sit inside the same roaming zone.

Contract length, cancellation and the practical bits

Monthly does not always mean flexible. A rolling one-month contract that you can cancel with thirty days' notice is genuinely different from a twelve-month deal that merely bills monthly. If your travel pattern changes with the seasons, it is usually worth paying the small premium for the rolling option.

Then check the boring details that cause the most frustration: whether the SIM is physical, eSIM or both; whether you can keep your existing number; how long activation takes; and whether customer support operates in hours that suit your time zone. A plan that takes three days to activate is no use if you fly on Friday. Confirm too what happens at the end of the month — some plans auto-renew at a higher rate once an introductory period ends.

A pre-commitment checklist

• List your top five destinations and confirm each is covered on the same terms.
• Find the fair use data cap and compare it with your real monthly usage.
• Check the roaming time limit if you spend more weeks abroad than at home.
• Note the per-minute rate for calls outside your inclusive bundle.
• Confirm the contract is rolling, and note the notice period.
• Check activation time and SIM format before you book flights.
• Ask what the price becomes after any introductory period.
• Test whether tethering and hotspot use are permitted, as some plans restrict them.

None of this takes long, and it is far less painful than discovering a problem from a hotel room abroad. Choose for the trips you take rather than the trips you imagine, and a monthly plan can be the simplest part of your travel admin.