Roaming allows a GSM phone user to make and receive calls using any other GSM network. This makes it possible to stay in touch more or less anywhere.
Choosing a Foreign Network
When you take a GSM mobile phone to another country, the handset will try to find its home network, and will probably fail. It will then scan for all the networks it can detect, and then decide which to use. Normally, this decision is left to the handset with the "Automatic" setting, but users can select a particular network.
The handset will choose one of the networks listed in the sim card's preferred list, if a network listed offers sufficient signal. Failing that, it will select any of the networks available, provided that it is giving a strong enough signal. Handsets are supposed to treat all networks equally if the signal exceeds a certain threshold, but in practice, they seem to go for the strongest. That's why the signal always seems strong at airports!
The Preferred networks list may be automatically maintained (as it is on Nokia phones) or the phone may allow the user to amend it manually (Motorola and Ericsson). Nokia users can use other makes of phone to do this maintenance if they want to.
If a network refuses access (for example because there is no roaming agreement with your home network) the sim won't try to use that network again, unless forced to by a user intervention (switching to Manual network selection, and picking that network).
This means that a brand new network may reject your access, and a year later, your phone won't try it again, even though roaming is now available. To fix this, manually select each network in turn when you visit a country, which forces the mobile to attempt to log on again, even if rebuffed last time.
Most UK mobile networks have roaming agreements with several networks in each country, so you are able to hop to another network if the coverage isn't good enough on the first. The registration procedure simply happens all over again each time the handset switches network.
Making and Receiving Calls when Roaming
The way that calls are carried and charged differs for incoming and outgoing calls. See Making a Call When Roaming and Receiving a Call When Roaming for more details of what happens.
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