My use case for Signal is friends and family, and it was easy to get everyone onboard because we all have each other's phone numbers already and didn't need to build a new list of contacts. It's a drop-in Android-compatible replacement for iMessage.
My phone number identifies my country, my address and my real name - even if I restrict the listing, it's tied to my credit card. It's tied to a sim card with separate geolocation data to the GPS tracking Google does; even if I active signal from eg a pine phone, the number is tied to a 4g base station.
Ed: and its tied to my current place of employment, too.
None of this is needed/wanted for my signal identity (for me or signal).
I could go out of my way to acquire a pseudonymous phone number, but I guess I'd have to be able to use it somehow - which seems pretty hard to keep anonymous. At the very least I'd probably have to pay for it.
Signal should be able to do better than PGP and five mix master hops of 90s-era anonymous email...
Or you get the old problem of those needing actual secure communication using terrorist@phreak.suspicious.net.ru and using signal just for "other" stuff..
Ed: note that this mostly about connecting with people on signal that otherwise might not have my number, than about (almost) random people that have my phone number discovering that I'm on signal.
You have to give up your anonymity to get one in many places.
"Just give your phone number to us, and don't worry, we won't share it with anyone!".
That exactly what Signal does.
Until they allow user-created ID's with no link to any identity - the above concern stays.
You don't need to use a GSM number, and you don't need to use the country code in which you live. The fact is, mostly anonymous phone numbers are available on the internet for use with Signal, and Signal (correctly) does not discriminate on country code or "type" of number. Any number that can receive phone calls or texts will do.
There are indeed countries that want to tie phone numbers to strong identity, but you can simply get a second number from a country that's not so hellbent on restricting access.
The number you're logged in to in Signal on a phone does not need to be the same number of the SIM card inside that phone. You can use any number you wish.