Once you are in Discord you have to join the #general voice channel, which you do by pulling your screen to the right or tapping the three lines in the top left:
Join the #general voice channel:
Once you have joined the #general voice channel be sure to mute your mic. Heavy breathing, background music, and extra feeback once there are several people on the channel can get quite annoying. It is possible to mute a particular user, so be careful not to do that.
Discord can be a bit finicky. It tends to kick people off periodically. If you have not heard anything for a bit, or no one responds when you talk, hang up (red phone) and rejoin the #general voice channel.