If you are working for a business, the standard way to inform caller that they will be put on hold is, "Please hold" or "please remain on the line." (not "hold on" or "hang on", which are very familiar ways to express oneself, in my opinion, and should not be used in a business, but rather reserved for friends and family, at least in the US).
It is more usual to ask the customer, "could you please hold?" or"Would you mind holding?"
Please remain on the line.
Your call is important to us.
Your call will be answered in the order it was received.
There are approximately 27 callers ahead of you in the cue.
Would you like to continue holding?