Disable incoming faxes during holidays

on your fax extension, just turn off the "take messages" option

To be clear, I want the faxes to not go through as "sent" for the sender, not just to stop our office from actually receiving faxes sent to us. Also, when I log in to the fax extension, it seems to still take me to the main line instead of just the dedicated fax number, and I don't want to disable people's ability to leave us a voice message during the holidays.

yes, if your fax line does not answer and does not take messages - it should not be received.  Obviously test this first.  If for some reason it does not work, then change the line type to voice-only, and take no messages.  Again, probably after hours, configure and test.  Let us know how it works out.