We are REQUIRED to provide the classroom phone number on the syllabus, on our classroom websites, etc.

It's just part of the current climate. We are to be accessible at all times. You've heard of "the customer is always right" of course, but "the parent is always right" is a close second.
Whether the phone is ALWAYS, NEVER, or "up to teacher discretion" to be set to "forward to voicemail" depends on the current administration and I have worked under all three situations. I encourage my parents to email and ask a question or leave a number if they want me to call them. Most do email and I reply as quickly as I can. The ones who call are usually the parents who don't have an email account, have a limited time available during the day to call, or have a child with a medical issue. Depending on the current administration's attitude, I have the option to:
- Answer the phone and make arrangements to call the parent back, usually during my lunch.
- Answer the phone and answer a quick question.
- Let the call go directly to voicemail during direct instruction and call the parent back when I have a chance.
I understand why some administrators want us to answer the parent's phone calls immediately. Some parents don't have the patience to wait for the teacher to return the call and call the office to complain, some within 15 minutes.

They would rather ask teachers to juggle one more thing than deal with angry parents.