What would cause one extension to not accept calls from other extensions?

Weird question. We have about 10 extensions and they can call each other no problem. However, extension 1420 never works. If I call it from my phone or another phone, it just says “Calling out (INV)” on my Cisco phone, then after 15 seconds it gives a busy signal and says line busy. If I call into my main line and on the IVR dial 1420 it transfers to it, it’s only from another extension in the office. No settings seem any different than any other phone, does anyone know what I could check or what might cause this? The actual phone at extension 1420 can call any other extension fine, it’s just receiving calls.

Just double checked it, the settings are 100% the same as my other extensions. When you call it you immediately get a busy signal and the phone says “ReOrder”.