Might be cultural, but I would prefer a neutral tone. The friendly tone gives some expectation of good result of the inquiry or of implication, which makes it worse when the problem is not solvable or not in the power of agent to solve - which many times it is - you don't call support for simple problems.
Of course I agree that "angry" is in most cases not appropriate, but still, I can see cases in which it might, for example, if the caller is really aggressive, curses, or blames unreasonably the agent, the agent could become angry. Training people that everybody will answer them "friendly" no matter their behavior does not sound good for me.
