I had many people like that at a place i used to work at but shall remain nameless so you don't feel like ur taxdollars are going to waste ;)
Anyway, as their supervisor their work reflected upon me, and I'll be damned if I'm gonna look incompetent. I always talked to new people and gave them my own interview to find out what their strengths/weaknesses were and what their background was. I then used this information to setup several OJT scenarios. Yes, I was breaking things but it wasn't in a mean way. I told them upfront that everything I did was for training and I wouldn't give someone something that would be impossible for them to fix. If a person didn't know alot about basic things like what hardware needs to be present and functional for a computer to boot, then i'd do something small like dislodge a ram stick just enough to where it wasn't completely obvious but still failed post. Once they figured out all those beeps meant something, i'd have 'em google up a list of beep codes for that bios, etc.
There are many ways to OJT people up to speed and be productive without being a complete a-hole or making them feel stupid. If your boss thinks that he doesn't have to know a whole lot about basic stuff because all he does is answer the phone, then you need to explain to the boss in a way he can understand. Even if a person's only job is to answer the phone, he's still the face of whatever department he's answering calls for. And believe me, people aren't going to say "wow that guy on the phone is stupid", they're going to say "man those people at the help desk are retarded." Not to mention that the person answering the phone should be smashing small problems right then and there so little things like an unplugged CAT5 don't get passed to the techs, thereby lowering their workload.
Either way, there bottom line is, you need to sit down and talk with someone, him or your boss. Hope that helped...
