call vs call on

call is a noun but call on is not a noun.

Word	Noun	Adjective	Verb	Adverb
call	Yes	No	Yes	No
call on	No	No	Yes	No

As verbs, call on is a hyponym of call; that is, call on is a word with a more specific, narrower meaning than call:

Other hyponyms of call include see, see, come by, drop by, drop in.

call (noun)	call on (noun)
(sports) the decision made by an umpire or referee
a visit in an official or professional capacity
a brief social visit
the option to buy a given stock (or stock index or commodity future) at a given price before a given date
a method of contacting a person by phone
a loud utterance; often in protest or opposition
the characteristic sound produced by a bird
a request
a demand
a demand for a show of hands in a card game
an instruction that interrupts the program being executed
a demand by a broker that a customer deposit enough to bring his margin up to the minimum requirement

call (verb)	call on (verb)
rouse somebody from sleep with a call	have recourse to or make an appeal or request for help or information to
challenge the sincerity or truthfulness of	to visit someone, usually for a short time
consider or regard as being
utter in a loud voice or announce
ascribe a quality to or give a name of a common noun that reflects a quality
get or try to get into communication (with someone) by telephone
assign a specified (usually proper) name to
utter a characteristic note or cry
send a message or attempt to reach someone by radio, phone, etc.; make a signal to in order to transmit a message
order, request, or command to come
indicate a decision in regard to
challenge (somebody) to make good on a statement; charge with or censure for an offense
declare in the capacity of an umpire or referee
utter a sudden loud cry
make a prediction about; tell in advance
lure by imitating the characteristic call of an animal
present for redemption before maturation
read aloud to check for omissions or absentees
order, summon, or request for a specific duty or activity, work, role
order or request or give a command for
give the calls (to the dancers) for a square dance
make a demand, as for a card or a suit or a show of hands
demand payment of (a loan)
pay a brief visit
call a meeting; invite or command to meet
make a stop in a harbour
greet, as with a prescribed form, title, or name
stop or postpone because of adverse conditions, such as bad weather