It is dangerous to use such messengers like Signal (or Telegram) with a real phone number.
Which I've done personally for many years. They won't tell you it's possible at the stores but they don't ask questions if you say you forgot your drivers licence at home.
Fortunately that's not the only identity separation you can create, which IMO is as or more important day to day than encryption and other technical solutions.
But nothing beats limiting government power through policy which seems to unpopular these last few decades.
The problem is this would both a) need to be used by Signal as the primary number ID b) your friend would need to know this separate number just for chat (unless you also use it for calls and other stuff then no biggie)
Of course this also kills new friend discovery, which is probably not needed by most.
I haven't tried using SIP on my phone app while simultaneously having a SIM number so I'm not sure Signal can be routed through an arbitrary number on your phone. But I could be wrong. Maybe someone else knows?
Here in New Zealand, I can freely purchase any prepay SIM without ID and use it straight away. Most, if not all dairies carry them too.
You can even purchase cards without ID that also work in Mainland China and aren't subject to the great firewall.
NZ is obviously loose due to large number of tourists, I reckon. So easy to get SIM cards, I've got 2 (spark and 2 degrees). US also requires ID to get SIM last time I was in bay area (2016).
Pretty sure they all do. I've used prepaid on all the major carriers and bigger MVNOs and I needed to give some form of ID to the signup process.
In Germany you now apparently need to show ID and provide an address, though no proof of residence so any address seems acceptable.
This is a flaw common to services that rely on phone numbers as IDs. In many countries, one cannot purchase a SIM card without showing ID (and the seller makes a photocopy of the ID to provide to the authorities). That means that there cannot be true anonymity. Know the phone number, know the person.
I am always baffled when people claim that PGP-encrypted e-mail is passé because it leaks metadata, when Signal and Telegram leak metadata too and, furthermore, metadata that can be immediately associated with a specific person in many countries.