are you talking about a specific race or in general ?
generally headphones are not allowed at any triathlon. personally I would not burden myself with carrying a phone.
you can have GPS, or watch ...no issues there.
also no problem with carrying your own drinks and nutrition, although usually the course supplies adequate aid stations for hydration, some stations have gels/powerbars etc (usually the run will be more supplied).
the race details on the site will say what they have and which aid stations (generally).