True, but it is still distracting when it rings and if you don't pick up it automatically plays through loudspeaker !! I don't mind if it's signaller, but really Control should be putting the phone down if I don't answer the GSM-R and not letting it get to that stage. They've got the option to send 'contact control' message.
Anyway, we digress. I say a watch is a must, as does the rules. Even if your train has one in the cab, what about when you're making your way to the train or on break. Yes you may have a phone which can tell you the time, but much easier to look at your wrist than take something out of a pocket. Also useful for gauging distance should you ever need to lay protection (- it takes about 15 minutes to walk 1.25 miles roughly although that also depends on the footing in the cess)