In many of today's offices, you can't get to a POTS line from a desk, as everything is going through a digital switch/PBX.

There is a place that you can find that POTS line. The Fax. It is almost always hooked into the POTS via the standard (US standard anyway) telephone plug line.

All you would need to do is unplug the fax from the wall jack and plug in an old fashioned phone. You might even have one laying around at home. Take it to work, throw it into you desk draw and problem solved without involving the boss.