
I don’t know what they’re even doing. TeamSpeak/Mumble is not a replacement for Discord. There’s no separate text channels in addition to the voice ones. It’s just a VOIP program. If you move from Discord to one of those you’re either in addition fundamentally changing your way of thinking or you’re in for disappointment.
For one there’s no “public communities” as with Discord. Here are the biggest servers from mumist.eu:

I don’t think they’re handling their current scale well just in Europe, even. VoLTE works on a limited number of carriers even in Europe. e.g. only T-Mobile is recommended by FP in Poland, but it’s just one of the four large telcos. I wouldn’t expand if I was at that point. Plus, they’re busy with making their support less horrible (see post on their forum) for now.
edit: damn fp6 doesn’t support any Polish carrier, a step down from FP5 https://support.fairphone.com/hc/en-us/articles/9779766652689-How-to-set-up-4G-and-Wi-Fi-Calling
Hey, maybe the real house flippers will play this instead of putting printed-but-looking-like-handwritten pamphlets into my mailbox.