Deaf or hard of hearing people can use the phone, even if they can't hear and understand voices on the phone.
In the US, and in many other countries, a free service called Relay is provided.
In the US, the service is paid for on a state by state basis using a small surcharge on all the phone bills. Typically this surcharge is only a few cents a month.
With that money, each state contracts with a relay service, which provides relay operators called communication assistants (CAs) who participate in a relay call by typing what the person you called says, so you can read it. The CA will voice what you type for the other person to hear what you typed.
Relay calls using the phone system only charge for portion of the long distance call between you and your called party. There is no extra charge for the call to the relay center, which may well be in another state. Some relay contracts also provide a discount on the long distance rates because of the speed differential of communication with typed words.
Each state has a series of relay numbers which can be called to place your call. Some states have also set-up "711" number which will go directly to a CA to initiate a relay call.
Relay services are used primarily by Deaf, deaf or hard of hearing people, but special relay services are also available for speech impaired people.
Relay services typically now offer a "branding" feature which allows them to know your preferences automatically when a call comes in from your phone number. For instance, if you are hard of hearing and using VCO, you may be "branded" to a relay center which employs CAs which don't have heavy accents.
Originally, relay required you to make a phone call using a TTY using either Baudot or ascii or a computer using ascii.
Now, relay also provides IP-Relay which allows you to make relay calls from any computer connected to the Internet. One good thing about IP-Relay is that the calls are completely free, since they don't use the phone system for long distance, but use the Internet instead.
If you can hear a little bit and speak, you can also use VCO Relay which allows you to hear the party you called and speak directly to them ... and the CA will just type what the other person says so you can read it.