To run the voice server these plugins need an open port on your Minecraft server.
Gilby's Voice Chat Reloaded doesn't need to be configured for this since it uses your server port by default. You only need to install it on your PC and the server.
Simple Voice Chat doesn't do this which means you need to configure the port manually.
Finding your port
The port in this example is 28487. Your port will probably be different!
Configuring the mod
To generate the configs you first need to install the mod and then start your server. After that, you need to stop the server again. It's important that your server is offline while you change the config because the mod will overwrite it otherwise.
If you're using forge, you can find the mod's config file by opening your world folder and clicking on 'serverconfig'. The file you want to edit is named 'voicechat-server.toml'.
For fabric that file can be found by going to files, then config, voicechat, and opening voicechat-server.properties.
Here you want to change 'port = 24454' to your server port. In this example that is 52713. Now you just need to start your server again and you can use the voice chat.