Nowadays, a majority of apps require you to sign up with your email or even worse your phone number. If you have a phone number attached to your name, meaning you went to a cell service/phone provider, and you gave them your ID, then no matter what app you use, no matter how private it says it is, it is not private. There is NO exception to this. Your identity is instantly tied to that account.
Signal is not private. I recommend Simplex or another peer to peer onion messaging app. They don’t require email or phone number. So as long as you protect your IP you are anonymous
I know telegram won’t accept voip numbers. I think I remember trying with signal as well with the same results. Clearly they attach enough importance to only having accounts with easily trackable devices like android and iphone devices that they are putting significant effort into blocking all other accounts.
You’d have to anonymously buy a preloaded sim and a burner droid phone to make the account. Its a lot less effort to use other more privacy friendly systems. Even more so if you’re making multiple temporary accounts, which is also an important part of reducing your trackability.