The cost of using a phone abroad varies from nothing to a great deal depending on the country, on the same plan and the same handset. The billing follows agreements between networks rather than distance.
Roaming runs on wholesale agreements
When a phone connects abroad it uses a local operator's network, and that operator bills the home network under a negotiated wholesale rate.
The home network then charges the customer according to its own retail policy, which may bundle, mark up or cap the underlying cost.
Because those wholesale rates differ enormously by country, retail roaming prices follow the same pattern rather than being uniform.
Regional agreements flatten some of it
Within certain regional blocs, regulation caps what operators may charge each other, which allows home-rate roaming across many countries.
Those arrangements are geographically bounded and fair-use limited, so a country a short distance outside the bloc can cost very differently.
Cruise ships and aircraft use satellite links that fall outside every such arrangement, and they are consistently the most expensive way to connect.
Data is metered differently from calls
Data is billed by volume and can accumulate silently through background application activity, updates and cloud synchronisation.
Many operators apply a spend cap that suspends data at a threshold, which prevents the largest bills but also stops service unexpectedly.
Incoming calls can be charged while roaming even though they are free at home, since the call is being carried to another country to reach the handset.
Local options bypass the chain
A local prepaid plan buys capacity directly from the operator at domestic prices, which is normally far cheaper than roaming on it.
Downloadable profiles on handsets that support them achieve the same thing without a physical card, and can be arranged before departure.
The trade-off is a different number for the duration, which matters where messages or verification codes are tied to the home number.
Reducing the bill without losing service
Turning off data roaming and enabling it deliberately keeps control of when charges begin, and offline maps remove the largest routine data need.
Checking the operator's rates for the specific country before travelling takes a few minutes and is the step most often skipped.
Keeping the home line active for calls while using a local plan for data is a common compromise on dual-profile handsets.