Not able to disable Airtel call forwarding

Call forwarding is enabled somehow on my number to a unknown number - 8809001019 and Airtel is not allowing to disable this. Please help me remove it, Airtel support is useless they say that call divert is not enabled and it is problem of my phone but I am getting same thing on putting sim in other phones too

See if Voicemail service is enabled on Airtel.

Yes, It is enabled but don't know how to disable it, sending sms STOP VMS to 54321 didn't worked

In the Airtel app, check under "Manage services", it should list all activated services including Voicemail. You can deactivate from there. You could also place a request to customercare to have them disable it.