At a previous company, I had a subordinate who would do one of the following, they all mostly seemed to work after 1-2 times:
- Leave a note for the first offense
- Hide the phone somewhere in the employee's workspace and leave a note to that effect (often in the file drawer in a new folder labelled "HOW RUUUUUUUUDE")
- For phones with a removable battery, remove the battery, and stack phone, cover, and battery like a zen stone arrangement in front of their keyboard
- In one case, use the administrative "all page" function to blast the very distinctive (unique to the company) annoying ringtone across all the desk phones. Subtle!
- Holding the cell between thumb and forefinger, run (not walk) feverishly into the conference room the offender is meeting in, and with sweat on beaded brow, hand over the phone like a precious baby, or bomb to be defused
The number one key is to have a company culture that doesn't tolerate that sort of rudeness – even having ringtones go off when you had the phone on your person was considered really taboo – certain managers just wouldn't continue with a presentation/meeting until the offender had left.
People catch on pretty quickly, if you treat it as a serious disruption. If any of the above sounds, instead, like it would get you in trouble if you tried it, then it's probably best to invest in better headphones, and/or a cellphone signal jammer (er, I mean, cell signal testing equipment).