You and your contact will both have to be on an internet connection for your call to start ringing.
If the call does not ring, it is likely that your contact is not on an internet connection at the time you placed the call.
If you can not hear your contact or your contact can not hear you, make sure microphone permissions are enabled and your firewall is not limiting or blocking Signal's access to the internet. iOS users, check the full list of iOS permissions. Android users, check your phone settings.