And how are you going to convince others to pay for Signal when there are many free alternatives, including WhatsApp, which most people already have and while not as privacy focused as Signal, does have end-to-end encryption. If Signal makes people pay for voice calls, they will simply use WhatsApp, regular phone calls, or whatever is free and popular at the moment.
The success of Signal came from being very low friction, privacy is the "nice to have" feature, at least for most users. But add friction and they will look elsewhere, Signal is not WhatsApp, it doesn't have enough of a critical mass to keep users on its network.
All that will remain will be a small core of cypherpunks and people who really have something to hide. This is bad because one strength of Signal is that it is a mainstream app, making it hard to single out "interesting" people compared to those who just use it because their geek friend told them to and they like the shade of blue.
They already do some of these, and some are less popular than others, but the key is to keep the essential features free and easy.
On Discord for instance, a free account is enough to cover all of most people needs, but you get a little extra by paying a subscription, and it is enough for Discord to be worth billions. Maybe not the perfect example since Discord has a critical mass, but no one wants to leave just because they don't have premium features (larger uploads, higher resolution streaming, flashy emoji) for free.
For Signal, it seems like just calling for donations is enough. They have a good image, so they can do that. It can actually be a solid business plan, look at Wikipedia, they get more than $100M a year doing that despite the controversy.