How does a person use a tty if the other person doesn't have a tty number?

They use a relay service. What they do it call a certain number with their TTY, and the person at the relay service calls whoever the person using the TTY needs to call for them. That way, the TTY user can use their TTY, and there's no problem with not having a TTY on the other end of the call. Also, many places such as doctor's offices have TTY systems, and a TTY phone number to call if you are using a TTY. Hope this helps! :)