You can get a phone number from https://jmp.chat/ if you like. The signup process can be done entirely over Tor.
If you don't use it beyond the trial, it's like the public payphone option mentioned by another commenter - someone could take your number. But if you choose to get a paid account (which, among other methods, can be acquired using Bitcoin, Bitcoin Cash, or a prepaid gift card purchased with cash), then the number will be yours. JMP is probably the most anonymous way of getting a phone number.
If you don't use it beyond the trial, it's like the public payphone option mentioned by another commenter - someone could take your number. But if you choose to get a paid account (which, among other methods, can be acquired using Bitcoin, Bitcoin Cash, or a prepaid gift card purchased with cash), then the number will be yours. JMP is probably the most anonymous way of getting a phone number.