Discord is the ultimate chat program for gamers, including those of the tabletop varienty.  The voice and video call clarity is excellent.  You can make “servers” for your games (like group instant message but with channels that have different topics), and discord has a wide variety of bots you might find useful.  You can even join large, world-wide servers just for talking about different RPGs, VTTs, etc.

Here are some of my favorite bots for any server:

• RPG Schedule — Schedule events and remind your players of them.
• Simple Poll — Polls!
• Starboard — Memorialize the hilarious stuff your players say in chat.
• RoomBot — For creating temporary play-by-post or voice role play rooms.

Discord TTRPG Bots

• Avrae — does dice rolling and much, much more for D&D 5e. It was acquired by D&D Beyond and full DDB integration is planned for the future.
• Sidekick — dice rolling, playing cards, and macros
• Dice Parser — full-featured dice rolling sytax based on Rolisteam
• Dice Maiden — a general dice roller with special support for WoD, Fudge, and WH40K rolls
• General purpose bots — some general purpose bots already have dice rolling incorporated.