call box vs letter box

call box and letter box both are nouns.

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

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

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

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

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