I received my renewed passport with my old docs about a week ago. On Jan. 20th I received two missed calls from this number. Calling back, using the number recorded under missed calls, resulted in "we cannot complete call as dialed". Has to be a scam.

It's legitimately the National Passport Center.  They block incoming calls to this number because all incoming calls go through their Customer Service Center: 877-487-2778.

If it is legit, why won't leave a message?

Never mind....it truly is NPC.  It just took a while for the message to appear on my phone.