Why do deaf people need a phone connection to use Next Generation Text (NGT) app?

The phone call enables deaf users who want to speak to the hearing party to do so.  Similarly, users with some hearing can listen to the other end of the call while reading the captions, and speech-impaired people can listen to the call and type what they want to say.

A purely web-based system would require a continuous data connection (3G or Wi-Fi) on mobile devices in order for users to know that they had an incoming call.  This could lead to battery drain and/or higher data consumption.

Similarly, for fixed lines, a purely web-based system would need the internet-enabled device to be powered up and connected to the internet in order for users to know that they had an incoming call.  With a regular phone call, alerting systems like flashing house lights and local pagers can be used.

For emergency text relay calls, location information can be used to find the caller in exactly the same way as any other emergency call.  Ofcom requires UK communications providers to make caller location information available free of charge on emergency calls.

 The phone call identifies the telecoms provider responsible for paying for the relay call, and also helps prevent relay fraud from being perpetrated (this has been a serious problem in the United States).