I am moving to Sasebo, Japan, for three years, and am hoping that I can use my AT&T Cel phone there with either Softbank or Docomo service without having to get a new phone. Anyone had any luck doing this? Thanks in advance! -Pat
If your phone is locked, it won't recognize Japanese SIM. You must unlock your phone, make it a SIM FREE phone. After that, you are free to use any services you want.
Regarding @jcaron comment, Apple Pay can be used in Japan just like in US. You need to change the region of your phone from US to Japan for it to recognize Japanese credit/debit/prepaid cards.
Please note that only iPhone 7 purchased inside Japan (model J/A) can be used as commuter card (Suica). https://support.apple.com/en-us/HT207154 (from @Zack Lipton's comment)