Why does the NGT app disconnect after my text relay call finishes?

This is to conserve battery life and minimise data consumption.  To receive another NGT call (i.e. if your phone is ringing), open the NGT app and click Join.  To make another NGT call on a mobile phone, open the NGT app, switch to your phone's dialler and dial 18001 followed by the number you want to call.  Switch back to the app and click Join.