[1] Central office code protection was once common in communities on provincial or state boundary lines.
[1] The entire 506-752 prefix is assigned to Aliant Telecom (NB) and serves Campobello Island, New Brunswick.
The 207-733 prefix serves Lubec, split between FairPoint Communications and a pair of competitors using number pooling.
This is a minor drawback in small cities with large rural area codes (as the numbers can easily be assignable in distant locations), but can consume numbers rapidly in larger centers such as St. Louis or Kansas City (where such protection must be maintained throughout most or all of the area code region, reducing its lifespan).
Eventually, the few remaining prefixes in area code 819 could not be assigned without requiring ten-digit dialing from Ottawa to Hull.