Here in the UK we have the old K6 , K8 and very rare K1 Red Telephone Booths. Many are converted to Little Librarys, Tourist Information, Mini Coffee Shop Outlet, or even just there as an historical piece, kept for local Nostalgia and links to the past, some even touching 100+ Years now. If this phonebox you are refering too was more towards this side of things, regarding a conversion use that meant people actually used the booth then i would say yeh give it a go! BUT if its more a phonebooth with an Open Wifi Router in it, with them being spread evenly over the city then i probably wouldnt put money on it. Have a go tho, its all about nuance!
Oh! And Trail Markers are the bread and butter of Niantic Games. They promote ALL 3 pillars.