I agree w/Trek - you'll have no need of phone while on the road, as the support crews have the communications set-up for you. I usually turn my phone off, but have it with me - just in case there's a reason I want to make or receive calls at a stop. Then turn it on each night to check messages and check in with folks. Our ESAR ride has had a "no signal" stretch in an area between the Finger Lakes and the Catskill mtns (we Washington-staters call those 'hills' tho!), so it was quite the funny scene at the 1st rest stop one morning, when people turned on their cell phones and got a signal, so everyone - riders, crew all - had their phones to their ear in every direction! Also, the power outlets on the poles of the covered shelter at the rest stop park, had phones/chargers hanging from every one! Same scene in the evenings, as our ride uses state parks and campgrounds, so the restroom outlets were shared and everyone just stacked up their stuff to be charged, so we could plug in the next after each finished...funny sight, but commonplace nowadays.
I agree re: Garmin also...you'll have signs, cue sheets, maps and crew to guide you along the whole ride route, so the Garmin isn't really necessary. It'll probably be most useful to you during training, but alot of things aren't really needed when you're riding the actual event (that is, unless you have and use the cadence and/or HRM functions).