I doubt that phones can receive signals underwater, making calls would be impossible, and texting unlikely even if they got a signal. I'd hate to have to hear someone else's choice in music on a dive. Many use their phones in place of real cameras, which has set amateur photography back by years.
Some phones are water resistant on the surface, like my old fashion Barrage, and have emergency GPS tracking. I can see the benefit of carrying one in a camera housing on a dive, just in case the diver goes missing, if he can retrieve it on the surface without dropping it and turn it on. Leaving it turned on during a dive with weak or no signal could drain the battery.