They can stand over you and make you turn on speakerphone if they want. 🙂 Heck, they could turn the speakerphone on in the breakroom while tapped into your call at your desk. It's their phone and phone service. You have no right to expectation of privacy in the workplace. (Except the restroom of course. And I think maybe that only applies within the stalls themselves.) They can monitor all your Internet traffic too, no matter what you connect to or who you communicate with (if they happen to record a plaintext password you send, too bad). They can stick a camera at your desk recording everything you do, or hide one doing the same thing.
And if you think you should have been caught by now, they're probably writing it all down as evidence to back up nailing you for whatever major thing you may one day do.