As a NON-vest wearer, but w/some experience dragging my meager belongings into hostile environments...
1)most phones these days are IP (Ingress Protection) rated(c ref) but IP protection does not account for pressure at depth, like dive watches rated in atmospheres. So you need more than a zippered pocket.
2)if you are on an FTX, the speed of access to a personal phone is not needed. I had heard, a couple of yrs ago of apps that you can use on your phone that may be useful, but I think because of software security, the usage of these apps is frowned upon.
3)you really don't want your X calling you at 3AM when you are sitting on an ambush site in sub-zero weather(for Canucks) Leave the speakers off..
4)All watches will break when abused. Short of someone with a job in the admin shop, FTXs are not where you experiment with expensive toys
Net Net: spend a couple of loonies on an appropriate Pelican box, hang-stuff where convenient.
My FOG $00.00002 USD , which is $00.000002 C Eh?
ref:
link: IP ratings for waterproof phones: what do they mean?