Ever since getting my 6+ I've found that call waiting is a bit hit and miss. Sometimes I don't get the 'beep' alert for a second incoming call at all, and I'm only alerted to the fact someone tried calling when they leave me a voicemail.
Even when not on a call, sometimes my phone doesn't ring at all and I just get the voicemail despite having a strong signal at the time. iOS8?
Yeah I believe extended call features don't work to well in iOS 8. I can be on a call and a second call comes in, try and sent to VM so it will stop ringing but that kills all audio on orig call. Not sure it happens every time but I have not ever been very successful with multiple calls since iOS 8. Sad that the main function of a PHONE doesn't "just work" every time and easily.
OP as for your answer, see the buttons on the phone while a call is coming in. They are pretty self explanatory how they should work anyway.