Flipkart — Not able to add Mobile number to my account - Technical issue

I have been using my flipkart account registered with my email: [protected]@gmail.com since so many years.
And, I had another account which was accidently registered with my mobile number, later got deactivated.
Now, I am unable to update my mobile number to my account registered with email – showing error as “ user already in use”.
In order to activate my flipkart pay later option, I have to add my mobile number, which is not possible now due to these technical issues. I have contacted their customer service several times, but no use.
Requesting you an immediate solution.
Email: [protected]@gmail.com
Mob: +91 [protected]