We are installing a centrally monitored, wired alarm system in our
house.

I noticed that the Network Interface device sits unprotected at waist
level on the outside of the house. Even more concerning, the phone
line enters and exits the box unprotected.

Other than paying for a radio link backup, what is the best way to
"harden" the phone line?

My understanding is that, if your alarm system is actually monitored,
then cutting the line will cause the alarm co. to react as if the alarm
had gone off, so hardening it is pointless. Is there some reason why
you don't trust the alarm company to answer these questions?