How to listen to voicemail with someone else on other line (iphone 4)?

Does anyone know you to acess you voicemail while your on the phone with something if you wanted them to hear a message? I can't figure it out.Everytime I hit the phone icon it just shows that i'm on the phone with the current person I can't get to my voicemail… Please help

Why do you ask this in the "Singles & Dating" section?