Try calling a contact and listen for a little pop sound right when you click the contact to call. If you don't hear it the first time, try again. Let me know if you guys have this little sound.
Put your phone on vibrate before clicking someone to call.
(THIS IS NOT DURING THE CALL, RIGHT BEFORE THE CALL HAPPENS - FROM THE TIME YOU CLICK THE CALL BUTTON ON THE CONTACT TO WHEN THE PHONES DIALER POPS UP TO CALL).
Make sure the phone is close to your ears as it's harder to hear further away