Not everyone works at a desk.
True. Moreover, even if you do have some company-provided means of contact for your non-desk job, that number may not remain the same from day to day, or even throughout your day. Case in point: as a staff nurse in a large, busy emergency department, I might be issued any one of twenty-four SpectraLink phones at the start of my shift. Each phone, ideally, goes with a job assignment (e.g. "RN #2, Blue Team"). Thus, if you change assignments in mid-shift, as frequently happens (and frequently with little or no notice), you turn in the phone associated with your old assignment and pick up a new. In any given twelve hours, I might switch between team service (we run four teams), Ambulance Bay, Float, Triage, and/or Resuscitation. Overhead page only works if you're in the department (not a given, particularly if you're on float service, as that means you're usually assigned to transport the real sickies to whatever flavor of Critical Care they'll be occupying next). Moreover, we are routinely admonished by Admin not to use the overhead paging system, as it's considered a noise hazard and detrimental to patient well-being. (See also: Press Ganey happy horsepuckey, but I digress.) Call forwarding isn't an option since, as previously mentioned, the phones are tied to jobs - you need to know you'll always be talking to the Ambulance Bay if you call that number, for example.
Given a system like that, the likelihood of an outside party being able to successfully get hold of you at work, without the assistance of multiple operators and several failed attempts, is pretty slim. Somehow, I don't see the staff at Sometown Elementary being able to reason their way through a parent contact sheet that says "On Mondays from 11A-3P, call 555-1212. From 3-7, call 555-1222. On alternate Tuesdays between 7A and 11A, call 555-1223, unless it's the third Tuesday of the month, in which case I'll be at 555-1335 until 1P," etc.
To that end, our administration runs a fairly benevolent policy on personal cell phones: they have to be silenced while you're on the unit, you can't make calls or use text-based communication in patient care areas, and the phone has to be completely off and stowed (i.e. even "airplane mode" doesn't count) if you work in a no-electronics unit (i.e. Electrophysiology, Interventional Radiology, Critical Care). Phone cameras, and any other form of recording device, are all lumped under the existing regulations on patient and visitor privacy. Luckily we don't have too many issues with phone impairment of productivity, as the nature of the job makes it quite evident when someone isn't working...