As per ICICI website, to register for mobile banking, one should either update the mobile number at the ATM or visit the branch.
1. When I tried to update at the ATM, it failed with error message "Invalid transaction or wrong account selected". I had no clue about this.
2. When I visited the branch, they said "You should call customer care for this". Visiting the branch was useless.
On calling customer care and being online for a few minutes, the information given is that my mobile number is linked with an ICICI pockets account, so I'm unable to register that number with ICICI bank account. He promptly delinked the pockets account and updated my mobile number with the bank account.
Why did this happen?
ICICI pockets is a wallet app that I downloaded on Android phone and used my mobile number with this app. I did not have a ICICI bank account back then. I did not use the pockets app, uninstalled the app and forgot about it. But number is in ICICI database and linked with pockets account. This explains the error message "...wrong account selected".
Since the mobile number was linked to the unused pockets account(and icici probably allows one number to be linked to one account only), my new bank account was not updated with the mobile number.
If you have used pockets account before opening bank account, you should either link the bank account and pockets account(if you have an option), or update the pockets account with a different number so that you can use your number with icici bank account.