Application of warranty, if bill is not available
Whether purchased bill is mendetory to cover warranty in the phone of Japan and using in India?
Especially when the warranty is available while entering EMEI
Given that an iPhone warranty is valid in the country you bought that device, Apple Stores or AASP might refuse to service your iPhone in India.
That said, if you want to know the Serial or IMEI number, see these instructions:--> Find the serial number or IMEI on your iPhone, iPad, or iPod touch - Apple Support
Regards
Giulio