You need to enable roaming to be able to connect through 3rd-party network provider.
You will be charged for
- in/out data usage,
- making and receiving voice calls, including receiving voicemail and retrieving it, and for also call forwarding.
- for outgoing SMS.
"Smart" people, like me, outright disable voicemail service and disable call forwarding to reduce "chargeable" vector.
If your device is capable of multi-SIM and you are not using it, sign-up for 3rd-party (data) SIM that is local to that country, then seitch to use data from that. Cheaper than you use data while roaming. Disable data of your MY SIM.
Also call your telco to discuss about NOT triggering auto roaming pass if you strategise to use foreign SIM later (heard that some MY telco immediately, automatically charge you so-and-so RM38/day pass if you happen to roam without one).
And also, once you roam, make sure you stick to one and only 3rd-party network that your own telco recommended. If you switch around network providers, you might get charged duplicate roaming passes for each. (this is my suspicion based on reading their TnC when i went out before, so i damn make sure manual detect network and stick to it.)