Boxes that provide fake phone lines often put out Data Noise in frequencies above voice frequencies.
We're talking about the fake phone lines you get from the cable company, the phone company who gave up on copper and is switching to fiber with a fake phone line, and some boxes from Internet providers who give you telephone service with Internet service.
You probably can't hear the Data Noise, but the real data from the CPU in the telephone or telephone equipment is sometimes affected by the Data Noise, so the phone equipment does strange stuff.
What strange stuff? It's going to be different on every piece of phone equipment. Maybe nothing, maybe stuff you've never seen go wrong before. There's no way to know.
Carry BOTH the standard and PLUS versions for testing. One may work, the other might not. There's no way to know until you try them!
The PLUS version uses a different filtering technology from our Regular version and covers lower frequency noise.
How do you know if Data Noise is causing your problem?
Hook up an oscilloscope across the tip and ring from the fake phone line box to look for the Data Noise and see the amplitude (how loud it is), which is very difficult to do in the field.
It's often easier to just carry a Data Noise Filter and put it in in-series with the mod jack coming out of the fake phone line box.
If you are selling / installing telephone equipment and some of it is going to be connected to a box with fake phone lines, keep a couple of these with you all the time (both versions). It will save a ton of time chasing your tail (and replacing the KSU, etc.). They're cheap and easy to try.