PO Box vs call box

PO Box and call box both are nouns.

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

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

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

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

PO Box (noun)	call box (noun)
a numbered compartment in a post office where mail is put to be called for	booth for using a telephone
	a numbered compartment in a post office where mail is put to be called for