Inadvertent roaming is when the signal from a mobile phone is picked up by a different mobile network with a better signal. This can happen inadvertently (without you being aware that this is happening). There are some areas in the EU where you can roam onto a non-EU mobile network that is not under the ‘Roam like at home’ rates. For example, if you are travelling in border areas, it is possible that you can connect to another non-EU country’s network and run up roaming charges inadvertently.
Another example is if you are travelling in the Greek Islands you might automatically connect to a mobile network in Turkey. To stop this from happening, you can disable the automatic network selection setting on your phone handset so that you only connect to the mobile network you choose.