Just because your WiFi is off doesn't stop calls coming through from overseas.
These call can come through via the normal routes.
Like all calls made via a landlines they will still come through
Calls and text messages go via the network (your sim card). You get charged by the minute for calls and so much per message for text messages) You might consider turning international roaming off (that is done at network level by your service provider, usually before you go ).. no data involved unless you get MMS .