All of the regions have voice chat enabled already. If you want to talk to other users in a region, you will need to toogle the voice chat on using this button (highlighted in red).
Once you have enabled voice, you can chat to other users by holding down the Right Alt button on your keyboard. If the region has Vivox instead of normal VOIP, you do not need to hold down Right Alt.
To enable or disable VOIP in your own region, click on the mini map in the right hand corner of the screen whilst in the region. You will then see a pop up like this:
The VOIP settings are Disabled, Enabled for owner only by default, and Enabled for everyone by default. You can choose which setting you want to use in your region.