The problem is with Optimum and IP Office.
Here is what I have seen before:

First, the Ring Detector circuit in the IP Office is very, very sensitive.
More sensitive then it really needs to be.

Optimum can/will/maybe send a type of Call Report data stream (like SMDR) about the previous call.

Or, it maybe a Message Waiting data stream (like Caller ID) related to their Voice Mail Service.

This data stream is loud enough (AC Signal) to be considered as a Ring Signal to the IPO.

I have also heard that it is a 30 Volt AC Signal too.

I believe that a KS Meter looks for DC and the Phantom Signal is AC, so that may be why you don't see it.

Try using your Butset (old Bell System are the best for this) on Monitor after you make a call and you may hear the data stream.

Solution is to tell Optimum to turn off all of their Call Reporting and Voice Mail services.

You can also try using a DSL Filter but it may block Caller ID.