How can I use my logitech c270 mic to call someone one skype?

Solution
If you've seen the green bar then it means it did picked up the sound. Make a quick test. Launch your Windows Start menu. Type "voice recorder" without quotes. You'll see a windows store trusted app. Open/click it. Click on the mic symbol in the middle and start speaking. Finish recording and listen. If you can hear yourself, the person in Skype will be able to hear you as well.
Right click on the speakers icon in the system tray and click on Recording devices. If you see some microphones are listed there and one of them has a green tick next to it (with a name like logitech c270) then it means that your webcam is already activated your microphone and it's available to use. Try to speak and see if the bars on the right side increases. (Greener instead of empty.)

Then it'll mean that you can make a voice call and just speak. It'll work. If it's not there, try to install the drivers for the device. (You can get it from http://support.logitech.com/en_us/product/hd-webcam-c270/downloads)
 
If you've seen the green bar then it means it did picked up the sound. Make a quick test. Launch your Windows Start menu. Type "voice recorder" without quotes. You'll see a windows store trusted app. Open/click it. Click on the mic symbol in the middle and start speaking. Finish recording and listen. If you can hear yourself, the person in Skype will be able to hear you as well.
 
Solution