Plug headphone not support with macmini I have a Macmini 4,1. I tried to use plug headset with mic for skype call, but it doesn't work.

I can hear the voice from the other end, but my voice can't be heard on the other end. I tried with two or three headsets, but results the same.

Then, a friend asked me to try using USB headset, I brought it from one neighbour and now it works fine.why?

Is it a problem of my macmini, or all macmini would behave like this?

Referring to the answer, I have to decide to buy a new USB headset or not.
Here are the specs for this mini according to Wikipedia. Note that this Mac (and most Macs I'm familiar with) has an audio line-in which is a high-level input, but no mic-in (low level). That means you'll need an external microphone amplifier between the mic and the line-in input. The USB mic is doing all of that for you - amplifying the mic and converting the audio signal to digital.