Relay UK
UK only. Relay UK is the national text relay service: you type, a Relay Assistant speaks your words to the hearing person and types back their reply, so you can phone a doctor's office, a bank or a plumber who would never answer a text. The service is free apart from normal call charges, and it runs through an app on phone, tablet or PC as well as on an old textphone.
Two details make it more useful than it first looks. A personal TextNumber lets family and businesses call you directly without dialing 18002 first, and in an emergency the app's 999 button, or dialing 18000, connects you to the emergency services with a Relay Assistant on the line.
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