If it worked before , than it is not a hardware problem .
The trunk is programed as D.I.S.A. trunk or it has as answer point an PBX phone ?
I advice you to switch another trunk with this one an see if the problem persist than you're sure that is a CDE problem .
If is a CDE problem I advice you to check the Interconect restrictions between the trunk and PBX phone (that are depending of sense and you can have from a PBX phone outgoing acces to a trunk but it can't ring the PBX phone).