Signal will let you register without a phone number, for a price

Signal's beta adds registration without a phone number via a one-time anonymous $3 payment not linked to the account. Two-factor authentication has been added for such accounts.

• Registration without a number available in latest Signal beta
• One-time $3 payment made anonymously and not linked to account
• Two-factor authentication added for accounts without a number
• Previously the number could only be hidden via username