I have both an EPIRB and a Satellite phone - I would rely on the EPIRB more than the phone if stranded because of the nature of the satellite orbits.
Satellite used by the phone system is effectively stationary, so it is always in the same position in the sky relative to your current position. It is also a pretty low angle in the southern states. So if you are stranded somewhere that prevents your phone seeing the satellite, eg in a gully or at the base of a cliff, you will never get a signal out.
Satellites used by EPIRB are in an orbit that basically goes around the poles, so they are constantly changing position relative to your current position, so one will always pass overhead at some time - it might take an hour or 2 to pinpoint the signal, but the signal will always be picked up if you can see any sky at all