One of my extensions only gives a busy signal!

Hey everyone weird situation. Running FreePBX of course, and I have all Cisco 7960 IP phones. All my extensions are identical settings wise. One of my phones which has extension 1420 can’t receive internal calls. It has a DID assigned to it and if someone calls that DID it rings on the phone no problem. However if I pick up say my desk phone and dial extension 1420 it just gives me a fast busy signal and the bottom of the phone says Reorder.

I’ve tried rebooting the phone, checking all settings nothing works. I could try to delete the extension and start again but I have it all configured and set up so just didn’t want to mess anything up. Does anyone know what would cause a busy signal on just one extension? It does it even if I unplug the phone so it has to be a settings thing. When I check the PBX tab in FreePBX the settings are 100% the same as any other extension, aside from showing that extension vs another one of course. Any suggestions?

No ideas?