If you are a SIMBA Telecom subscriber and intend to data roam in Japan. This is something that you need to take note.
To roam successfully, you must ensure that you are have a VoLTE compatible phone. This is a requirement of KDDI. If you have been using a phone with the SIMBA Voice app in Singapore.
Of course, you would also need to adhere to the basic requirements of setting the correct APN etc.