As an assistant working with a phone, your job **is** to screen calls. People expect you to be the "*1st line of defense*". Most probably, though, they will try and go over your head.

That's why you need to have a script ready, and stick to it. You can:

1. answer the person if the question is within your knowledge/duty (and entitled to do so).
2. redirect the call to the right person/service.
3. schedule appointments.

Either you boss gave you a list of people he will answer calls from, or he didn't. When he didn't, make sure you have:

1. the name of the caller
2. the reason for the call
3. ask for the message

All this has to be done in a very professional and diplomatic way: show them that you're willing to help them, but it has to be **your way**, as you have a job to do, and a way to do it, as expected by your boss (and also a script to follow, but keep this part hidden from them, as it's none of the curstomer's business). Don't put them on hold, or ask to wait for you to check wether or not boss is available. Lead the call. Be nice, but **you** ask questions, and they have to understand it's done in order to help, not to dismiss.

Make sure they feel like you're the best person that can help them at the moment, and that the way to help them is to get this information, and make sure the boss/right person will get it, so they can get back to them.

All this can be achieved once you've coordinated with your boss, got the right script, and trained an bit. A quick internet search with the title of your question  (*best way to screen calls*) will lead to plenty of useful ressources.