Det ble vel ikke sendt nødsignal fra ubåten, det var vel mer det at de mistet kontakten.
Jeg har ikke lett så veldig, men har ikke klart å finne noe info om hvordan kommunikasjonen mellom ubåten og skipet skulle foregå.

The Titan would have had an acoustic link with its surface vessel, set up through a transponder (a device for receiving a sonar signal) on its end, and a transceiver (a device that can both transmit and receive communications) on the surface vessel.

This link allows for underwater acoustic positioning, as well as for short text messages to be sent back and forth to the surface vessel – but the amount of data that can be shared is limited and usually includes basic telemetry and status information.