. possibilité de couper toute la 3G et les connexions data avec l’appli APNdroid ou en changeant l’APN (Access Point Name)
ou
. désactiver le roaming
Go to Settings, Wireless & networks, Mobile Networks and then uncheck the box for Data roaming (connect to data services when roaming)
NB : je ne sais pas si cette manip est une méthode fiable à 100% pour avoir 0€ de hors forfait à coup sûr

Si on veut garder la 3G pour faire un peu de roaming raisonnablement(*), mieux vaut désactiver les synchronisations :

– désactiver les données en arrière plan
Go to Settings, Accounts & sync and then uncheck the box for Background data. If you had Auto-sync enabled, that should grey out as well.

– Disable location tracking by Google Search
Go to Settings, Privacy and then uncheck the box for Use My Location (Use My Location for Google search results and other Google services)

Disable automatic date and time update
Go to Settings, Date & time and then uncheck the box for Automatic (use network-provided values). Make sure to change to local time of the country of travel.

Disable Google Search settings personalization
Go to Settings, Search, Google search settings and uncheck the box for Show web suggestions (show suggestions from Google as you type). The Search history (show personalized search history for your email address) option should be greyed out.