Unable to release an individual line during a 3-person call.

When I get a call for one party and tried to add a call to the current call, the third party's call goes to voicemail. I am not able to release just the voicemail call. I would have to hit the hangup button which in turn releases both calls. My intension is to have 3 people on the line and when I am finish with one, I can release them and continue my conversation.