PLSB. Private Line Service Board.  This was the testboard AT&T devised for testing Special Services telephone circuits leased by large corporations.  There was probably an even split between voice and data, with a total of about 35,000 circuits.  Our job was to answer customer trouble calls and get them fixed as quickly as possible.  We had 30 minutes to clear the trouble out of our central office (STC), and an overall target of 2 hours to have the circuit back in service. 
We handled a myriad of circuit types, from simple Foreign Exchange (FEX) to four wire dial, two-point data, and multi-point data, both with our Western Electric modems (we called them Data Sets then) and customer owned modems (called CPE at the time). 
The testboard, as we called it, or more properly the testroom, was truly a beehive of activity, with perhaps 20 people testing circuits, answering incoming calls from customers and other STCs,  It was an atmosphere where those of us who thrived under high pressure, noise, and a disarrayed place found our calling.  Many of us stayed with the company and over time got promoted into management, ultimately becoming 1st, 2nd, and 3rd level managers.