call box vs PO Box

call box and PO Box both are nouns.

Word	Noun	Adjective	Verb	Adverb
call box	Yes	No	No	No
PO Box	Yes	No	No	No

As nouns, call box and PO Box are synonyms defined as:

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

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

call box (noun)	PO Box (noun)
booth for using a telephone	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