Simple Voice Chat
Simple Voice Chat is a robust Proximity voice chat mod for Minecraft that's available for the vast majority of modloaders for modern versions of Minecraft, making it a powerful way for players to connect to each other in-game.
The mod needs to be installed on both the server and client in order to work, but clients/players without the mod can still connect to a server that has it installed. This means that you can install Simple Voice Chat on your server without having to worry about the players who don't want to use it, they won't be affected!
Installation
You'll need to start by downloading the latest release of Simple Voice Chat for your mod/plugin loader and Minecraft version, then uploading it to your server's mods/plugins folder using the 
File Manager or SFTP.
After uploading the file, you'll need to restart your server for the initial configuration file to be created.
If you're running a network behind a proxy like Bungee or Velocity, you'll need to install/configure Simple Voice Chat on each of your backend servers separately.
Configuration
Before starting this step, make sure your server has a secondary port open dedicated to Simple Voice Chat. Read Requesting Additional Ports for steps on how to to request a port.
With a new port requested and allocated, you'll want to stop your server, then open Simple Voice Chat's server config file and change the port option to the port that was just allocated, which you can find in the Allocations Tab.
Now you can start the server and then you'll need to install the mod on your client and log into the server to test it, or use the /voicechat test <PLAYERNAME> command on a player that is logged in and has the mod installed on their client.
If it's still not working for you, check out Simple Voice Chat's troubleshooting guide.