> 2.2 | Hardware Requirements

> 2.2.1 | Connecting TTY Equipment to a Mobile Device

> To establish a reliable connection between a TTY and a mobile unit it is necessary to use a TTY/CTM conform modem. It allows a reliable transmission of text characters. The CTM performs character encoding and protection against channel influences. Additionally, the CTM signal is periodically suspended and the output is muted in order to avoid a condition where any voice activity detectors inside the cellular phone system might classify the modem signal as non-speech.
> Most TTY devices do incorporate a CTM modem (internal modem). To connect the TTY device to the mobile unit (customer application) the use of a 2.5mm jack which is usually part of the TTY is recommended. If the hardware of the customer application is not suitable to connect to the TTY or an external modem must be used it is necessary to assemble a TTY-to-CTM adapter.