Iphone call forwards when using phone and someone calls

So my mum recently called my iphone, she uses a samsung i think, and I was using it at time and was on the skype app typing, but then she called at the same time and she said that her call stopped and a voice answer said that I was online or unavailable to pick up a call, but then she called again straight after and it went through. Is this true? If so how can this be turned off? If its locked she can call, and even if Its unlocked it rings, and ive tried it with my brother while it was locked and when i was using an app and it went through.