Did your son have roaming enabled on his device?  Was the device compatible with the roaming partners?  Do you have international roaming enabled on your account?

We have Simple Choice Family Plan with 4 lines.

All 4 lines had “International Long Distance Blocking” ON and “Block Charged International Roaming” ON but “Block International Roaming (including Charged International Roaming” OFF.

Before flying to Tokyo today, my son upgraded to iPhone 15 and also bought “15GB International Data Pass” for his line.  When he got to NRT airport, he could not reach me. He had to use WiFi and we were able to text via my iPad iMessages as well as FaceTime, and my Android Google Messages.

For his line, I then turned “International Long Distance Blocking” OFF and “Block Charged International Roaming” OFF and kept “Block International Roaming (including Charged International Roaming” OFF to see if we can communicate without WiFi. 

1. At this point, I don’t know if we can communicate without WiFi.
2. Why does "SCL UNL TT FAM 4 Lines incl" appear as an active add-on now when it did not appear before? And what does the “TT” mean?  

Has roaming been turned on in his device's settings?

I am not sure. I will have to ask him. Do you know the step sequence in iPhone settings to turn on roaming?

I found the answer on the internet:

Settings -> Cellular -> Cellular Data Options -> oggle Data Roaming ON

Thanks

It should be under Settings, Cellular, Cellular Data Options.  Data needs to be on for roaming for VoLTE to work, which should allow calling to work.  

He said, “it’s on, it is just slow”. thanks, syaoran.

Thank you @syaoran for suggesting to check the roaming setting on the phone. 

To answer the other question, @tmn153 SCL UNL TT FAM 4 Lines incl - is the plan code for his line. 

SCL = Simple Choice Line

UNL TT = Unlimited Talk and Text

FAM 4 Lines incl = Family plan with 4 lines and this is included in that four-line count. 

I don’t know why it would not have been showing on his line before now, but I wanted to make sure I helped define what it is. 

Thanks, HeavenM, for explaining the abbreviations.