Gee - finally you guys are talking about a subject where I actually know something There are a couple aspects of using a cellfone/smartfone onboard that don’t seem to have been identified here.
First, a fone will connect to only one system at a time, so if you implement a “micro cell” system onboard for internal communications, you would want to lock all your fones to that system. This would mean that any connectivity to the outside world would have to be provided by the ships system - you couldn’t simultaneously also use outside world infrastructure to make calls and move data (Internet, email, etc.)
Second, “Micro cells” are how European airlines provide inflight cell service, so the technology is well-established, as are ways of interconnecting them with the outside - not cheap or maintenance-free, but do-able. In the US they are prohibited by law, so not operative here - this prohibition is only for aircraft, however.
Third, if these systems are strong enough to work throughout a big iron ship, they’re gonna have a footprint outside the ship as well - and this means that whatever country you are docked/moored in will have an interest in making sure you don’t interfere with their own systems.
In connection with the last, I am curious regarding the UHF handheld radios you are using aboard - do they really work from topside to below decks and way down in the engineering spaces? Do you have onboard repeater systems to enhance their coverage? My experience in metal structures leads me to believe that there would be coverage issues with UHF devices (such as smartfones, which are quite low power) on large (metal) vessels.