It could be a few things, but yes it could be a bad TN, but more likely it is wiring or a bad phone. If it keeps happening to the same phone, replace it. If it still happens then check the wiring/and or the TN at that point.
I have also seen a bad ground cause phones to disable on an Option 11.
JohnThePhoneGuy
"If I can't fix it, it's not broke!