letter box vs call box

letter box and call box both are nouns.

Word	Noun	Adjective	Verb	Adverb
letter box	Yes	No	No	No
call box	Yes	No	No	No

As nouns, letter box and call box are synonyms defined as:

letter box and call box: a numbered compartment in a post office where mail is put to be called for

Other synonyms of letter box include PO Box, POB, Post-Office box.

letter box (noun)	call box (noun)
a private box for delivery of mail	booth for using a telephone
public box for deposit of mail	a numbered compartment in a post office where mail is put to be called for
a numbered compartment in a post office where mail is put to be called for