"I have sufficient balance maintained in Paytm for a whole year and once a year I add to the Paytm balance using my US AMEX card."
Aren't foreign travellers now restricted from operating Paytm accounts?
I believe there may be some new restrictions enforced now but when I set up Paytm a few years ago it was registered to one of my Indian Airtel numbers and so the account is still valid and I access the account with the phone app on the phone which Paytm was registered to and access the account via the Internet. No authentication is required such as an OTP when I access the account using the original phone. I've made three top ups to Paytm this year, once in January 2019 while in India and two in February 2019 while in the US and the amounts were charged to my US AMEX card. There is sufficient balance in my Paytm account to get me through for another year.If I ty to access the account through a different phone then an OTP is required and so I use the original registered phone to do my top ups. My guess is that Paytm uses the phone's IMEI to verify the login.