(picture from this Flickr page)
Breakfasting at a local diner this post-Thanksgiving morning, my niece Ashley pointed to a sign on the door and asked "why do some sings say 'please call again' when they mean 'come back again'?"
Helluva question Ashley. First, WordNet lists a large number of synsets (meanings) for the word call (13 noun synsets and 28 verb synsets). It's a highly polysemous word. The relevant meaning for this use is "to pay a brief visit". Lacking access to the OED (the price I paid for leaving the academic world ... sigh), I used instead the Online Etymology Dictionary (and trust that it is relatively accurate) which lists this entry for call: "From Meaning "to visit" (M.E.) was literally "to stand at the door and call;" sense of "a short formal visit" is from 1862; caller "visitor" is from 1786."
So, why has this otherwise outdated meaning persisted in the retail customer service frame? I suspect it has to do with politeness. This older meaning evokes a friendliness, even a neighborliness which encourages customers who have missed the hours of service to not be offended. Something like that.