What is 711?
Dialing 711 from a payphone connects you to the Telecommunications Relay Service (TRS). Per the FCC.gov web site:
TRS is a service that enables persons with hearing or speech disabilities to communicate by telephone with persons who do not have such disabilities. The payphone relay program covers local and long distance calls.
Dialing 711 to access TRS is free but calls made through the service are billed to a credit card, calling card, or prepaid calling card.