Ok so first thing is make sure that you don't have international voice or data roaming add-ons enabled in self-serve. Then you can turn off the option to roam in your settings. Another thing that'll work simpler even is to keep your phone in airplane mode but I like to go with multiple redundancies just in case. Or even take your SIM card out of your phone. You can use a local SIM card where you are going or rely on wifi. Also you can go into airplane mode and then turn on wifi if you plan to use it and everything else will stay shut off.