Had to do that at my last job...it sucked. Only good thing is that you can "accidentally" drop the call and hope somebody else gets them when they call back (or even better, they don't).
Internal helpdesk, people generally won't be rude to you because they may actually see you in the hall or come down in person for assistance. Anybody that tried to yell at me on the phone would get the if they came down and asked me for help with something. Plus, like you said, you can tell their supervisor.
Only thing that sucks about internal helpdesk is when you get a call from an actual supervisor (or even worse, some big time person like the CIO/President/Executive Director). They'll pretty much expect their issues to be top priority and you end up kinda pressured to fix their issue asap. But usually they have somebody else call for them so it's not too bad.
this is so true to the 9th degree. Especially when a manager wants reps to be skilled in exony/uc