Staff the phone in the last week of December
The stretch when most offices are effectively closed is the stretch when a lot of family decisions get made.
Holidays put people in the same house for extended stretches, often after a difficult few months. Whatever your view of why, check your own records for late December and early January inquiry volume before deciding how to cover it. Do not take a national statistic on faith, and do not take ours either, because we are not going to invent one.
If your log shows what most firms' logs show, a skeleton rotation across that week is worth more than an extra month of ad budget. Someone answering live, with authority to book a consultation, not a service taking a message.
Set expectations in the outgoing message if you cannot staff it. A message that names a specific return date and offers a text option holds a caller better than a generic closed for the holidays.
Whatever you do, log the calls. A week you did not measure will be argued about for years.