If the 7100 trunk circuits are indicating ringing with nothing connected to them and the system performs flawlessly elsewhere it is almost certainly conducted EMI/RFI getting into the system, and not necessarily off the CO lines.
It could be something getting in on the mains supply due to either bad/missing grounding or interference. Is the UPS you are using online or offline - the latter pass the mains straight through when not operating so may not be doing much to filter mains interference.
Alternatively it could be interference being conducted back in on the station connections, are the DLM/SLM modules on the same UNI as the trunk module.
