Try the old phone tech trick.Open up the wires betwen the gateway and the PBX. Put a volt meter betwen the ground of the Cisco and the E lead. It should read some volts DC. If there is no battery coming out, there is nothing to be gounded by the PBX.
Once you located the battery (E lead out of the gateway), listen for dial tone with a buttset on TX pair and ground the E lead with a clip. The LED on the gateway should lit, dial tone should be transmited out on TX pair and on status of gateway that port should go busy.
Repeat procedure , but looking towards the PBX.It should work the same way.That will give you a direction to start looking for the faulty party.
Advanced troubleshooting: Located the E lead of the gateway (voltage present when circuit is idle).Put all the wires together.Put volt meter in paralel from ground to E lead. Using the PBX,”seize” or try to dial out on that port. Voltage should drop to almost cero.
Other things: Did you cross the wires??? Audio TX from gateway goes to RX audio of PBX, and vice versa.
Did you reverse E and M??? M of gateway goes to E of PBX and viceversa.
Did you tied the grounds of both equipments together??? Most terminals use common electric ground, but some others have a separate ground for signaling , also labeled “common” or Signaling Ground.
Hope that this help. If not at least it will keep you REALLY confused for a while.