Disconnects current call when a 2nd caller calls. Why is it doing this?

[AC Question]

Disconnects current call when a 2nd caller calls

I'm on a call and receive a 2nd call. Before I can answer my 1st call is disconnected and I'm speaking to the 2nd caller.

 

[ManiacJoe]

Re: Disconnects current call when a 2nd caller calls

Which phone?
What version of Android?
Which carrier?