Signal is not secure. Just ditch it. There are too many open and secure messaging/voice protocols that do not use telco (except via bridge services) already. There are even proprietary Big Tech things like Teams™ (use Matrix instead, there is even a bridge).
Discussion
I don’t disagree but just getting normie family and and friends off of straight SMS is SUPER hard. Personally, no one in my personal life is willing to make another switch beyond signal when just that took epic cajoling. This feels like a “don’t let perfect be the enemy of good enough” moment. I’m with you in spirit though. For BTC only people chats its probably fine.