How much data consumption for Skype audio call?

This was yesterday.

Audio call for a period of 47 minutes consumed only 21.8Mb of data. This was on a 4G network.

UPDATED: 12th July 2016

Audio call for a period of 41 minutes consumed 144Mb of data. That's weird right...

UPDATED: 19th July 2016

Audio call for a period of 1 hour 46 minutes consumed 170Mb of data. What???

UPDATED: 20th July 2016

Audio call for a period of 8 minutes consumed 45Mb of data. I don't understand.