We have a Nortel Networks T24 phone with multiple lines on it and 2 of them are showing like it's in use but no one are using them. I will like to know if there's a way to cancel them.

Exactly, a transfer is what usually causes this. when you transfer an incoming call to an external number, that uses both the incoming call's line and the outgoing cal's line. (2 lines). Since you don't hang up the call when you transfer, you rely on some signal from the phone company to know when those two parties are done talking. This is called "disconnect supervision." Usually, as a human, when you hear the dialtone, you know to hang up. Your PBX can't "hear" a dial tone and relies on a signal from the phone company to "hang up" the line.

If both of your lines are stuck as used, that signal has never come, or was not interpreted.

The short term easiest fix is to reboot the PBX. But to prevent it from happening again, you either need to ban internal-to-external transfers or configure disconnect supervision. Exactly, a transfer is what usually causes this. when you transfer an incoming call to an external number, that uses both the incoming call's line and the outgoing cal's line. (2 lines). Since you don't hang up the call when you transfer, you rely on some signal from the phone company to know when those two parties are done talking. This is called "disconnect supervision." Usually, as a human, when you hear the dialtone, you know to hang up. Your PBX can't "hear" a dial tone and relies on a signal from the phone company to "hang up" the line. If both of your lines are stuck as used, that signal has never come, or was not interpreted. The short term easiest fix is to reboot the PBX. But to prevent it from happening again, you either need to ban internal-to-external transfers or configure disconnect supervision.