Why am I getting the error message Call in progress on Viber while I was talking with someone?

[iMore Question]

"Call in progress" appeared on viber while I was talking with someone

Tonight I was talking with someone on viber, and suddenly above the duration call there was a shout that said "call in progress" and what happened is that the person with whom I was talking, was on hold. So he thought that I was in on another call, but I wasn't. Why does that happen ? Please help me!!!!