Calls from US numbers to US numbers incur no charge for the person placing the call no matter where in the world the receiving telephone is located.
The person will not get charged for texting you since it is still a US number that they are calling. Trust me I have dealt with this many times it will just count as domestic use on their end.
1) The call will redirect to voicemail. The caller won't pay anything extra but you will.
2) If they send you texts they will be charged whatever they normally pay to send you texts. If that's zero, it's still zero.
Your callers don't pay anything extra to call you when you are roaming because you pick up the tab for that. If you want to avoid being charged for incoming calls, redirect all your incoming calls to voicemail.
P.S. Your phone needs to have at least UMTS band 1 (2100 MHz) to roam in Japan.
Hi kirakira333 and thanks for posting!
That's awesome you're lookin' out for the folks keeping in touch with you while you're traveling. The people calling you would need to double check this with their own carriers, but I believe drnewcomb2 is spot on. I do hope you get the answer you're looking for.
Thanks, drnewcomb2 and everyone!
If any of my relatives/friends currently don't pay anything to send me texts while I'm in the US, does that apply also while I'm in Japan?
Also, will I only receive voicemail and text messages when I turn airplane mode off?
I have a Galaxy S7
I know of no way that the caller's phone in the US ever charges anything differently for calling someone's US number when they're roaming abroad. Receiving texts costs the same in Japan as in the US. If it's free here, it's free there. Current T-Mobile plans don't charge for sending texts while roaming. "Airplane Mode" needs and asterisk. You can turn Airplane Mode on then switch WiFi back on and make and receive calls and texts via WiFi, including checking your voicemail.