3.) Это почтовый ящик г-на Пупкина. Пожалуйста, оставьте сообщение или позвоните по этому номеру снова позже. Спасибо за ваше понимание.
In my (US-based) experience, doctor offices & the like have a place where you can fill out if they can leave messages or not (and some even have asked detailed messages vs callback messages), so that's my perspective! And that I can set different things for different numbers. So my cell phone they can leave detailed messages on any matter, my mother's house phone is detailed from billing only and otherwise callback, her cell phone is callback only. (And they also aren't allowed to tell her anything other than billing, so even if she does call them back about anything else! That was just a measure set up when I lived with her one summer & had no cell reception at her house.)

Some people, even most people, would always rather just speak to a person first, rather than listen to a recording of your office directory or department extensions. If you want to offer the operator first, try this:
A busy greeting should tell callers that your phone lines are currently in use. You can direct people to wait on hold or leave a message. For example, “Hello, you’ve reached [company name]. Our representatives are currently helping other customers. Please stay on the line to speak with the next available team member, or press one to leave a message and we’ll call you back.”