If the phone company is directing the data to another party besides those that are on the call then the Secret Service would never know. Unless they had access to all that routing. Which isn’t on their end.
It’s all VOIP and digital.
Well, I won’t argue with you .. except to say .. when a Secret Service person says the automatic sweeps of Trump Tower would have exposed any wire taps .. I tend to believe him - especially when he worked in the WH - and would certainly know what he’s talking about.
There are listening processes we may not know about - for the reason that it’s “secret”.