Personal — Line appearing busy

Address:

personal

Hi morning
I want to understand why would a number appear busy when the person was actually not on a call.
The message was clear stating that "the number that you are trying to call is on another call, please call later".
When would such a secario happen?
Would it happen if the person is not on a call and your automated message says the above..
I need to get this understood, because if in reality the person was not on a call and your message meant that the person was because the line appeared busy, then it is very very misleading.
Please help me understand in which scenarios, when the line appears busy, if would actually mean the person was not on a call.

The whole thing is :
I tried calling a number this morning and it came busy and i got the above message everytime i dialled, that the person is on a call. But this person says that he was not on a call at all. Can you please help me understand this, becuase if airtel simply gives out this message, then it is creating a rift between a couple. Hence i need to sort this out.

Appreciate your support in clearing the mislead, please.

Thank you.
Kind regards
Vera