Voicemail setup for India

I was able to start the voicemail service for my Airtel number in India. Now when I press the voicemail icon on the phone app it say error performing request, unknown error.

I have tried the following 2 suggestions-

1. Dial 5005*86*52555#. This did not work. I get the same error.
2. Reset Network settings and reboot phone. This option also did not work.

I am not sure and I cannot find any information if Airtel in India provides visual voicemail.

The voicemail number is 52555 which works when I dial using keypad.

how can I change the default voicemail number that is programmed in my iPhone XR?