May also be called Multi User Collective Kingdom or Multi User Construction Kit, depending on who you ask. The most common is Fuzzball MUCK, an open-source project. Two popular MUCKs are Tapestries and FurryMUCK. It is a text-based roleplay environment simmilar to a MUD but without the requirement to kill things and level.
A person who works in a call center, usually a lifer who has --for reasons unknown to the common man-- not been fired yet and has been deemed somehow superior to other call center agents. His job is either to rove or answer the mentor line. Roving mentors walk around the call center waiting for desperate agents to flag them down, sometimes involving the embarassing process of waving some sign or doing a chicken dance, in order to get help with resolving a customer issue. Mentor line mentors take calls from other call center agents and attempt to talk them through customer issues while the customer is on hold. You can't call the memtor line unless you have someone on hold, but you can get around that by dialing someone else, dialing the mentor line, then hanging up the first dialed line.