Ce qui suit me semblerait tout à fait approprié:
What follows would sound appropriate to me:

Puis-je vous demander de patienter un instant ?

Ou encore:
This would also work:

Puis-je vous faire patienter un instant ?

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?

Usually, a simple:

“Un instant, s’il vous plaît”

is more than polite enough for your need. For more politeness, you can state the reason before you use the previous form, for example:

“Je vais chercher M. Paradis pour vous, un instant s’il vous plaît”