He could have a device manipulates the ANI, and fake the caller ID make he seem like he's calling from another number
9 seconds sound clip from the In the Line of Fire movie soundboard.
You can hear this line at 00:27:22 in the Blu-ray version of the movie.
Quote context
[...]
- I was so moved by your honesty. The world can be a cruel place to an honest man.
- What about you, Booth? What's your story?
- I'll talk to you again soon. It's nice to have a friend.
- This device, where would he get it?
- He could build one if he has knowledgable in digital circuitory.
- But you can't you trace the call?
- We could run a comparity check.
- If he was online with digital switches.
- You mean, you can trace the call?
- Not if he's on an old twisted para-analog line, no.
[...]