hang on is informal:
hang on
2 informal wait for a short time: hang on a minute—do you think I might have left anything out?
• (on the telephone) remain connected until one is able to talk to a particular person.
-- New Oxford American Dictionary
Also, when speaking on the telephone, hang on can easily be confused with hang up.
To maintain a formal tone, use:
Please hold.
The on is not necessary:
please hold
Please remain on the telephone line until someone is available. Please hold while I see if Mrs. Smith is available. A: "Hi, is Jane there?" B: "Please hold."
-- Farlex Dictionary of Idioms