I would like to know the proper way to ask a customer if its alright to put them on hold while I search for the person or to hold until the person they want to speak to is not occupied.
If you just want to pick up the phone, tell them quickly "don't hang up", and put them on hold (e.g. a receptionist handling several simultaneous brief incoming phone calls), I've heard people say something like,
Cabinet médical, ne quittez pas. (or, possibly ne raccrochez pas)
If you want to ask them whether they're willing to wait quite a while, after you've talked to them and found out what they want, perhaps you'd say,
Voulez-vous attendre quelques instants? (or) Pouvez-vous attendre quelques minutes?