for usage Telstra did a test for face time, for both video and voice only calls:

"Telstra estimates a 10 minute VOIP call on a mobile would use about 6 megabytes [MB] of data while the same video call would use about 300 MB. But another VOIP app called Viber estimates its app uses about 2.3 MB for a ten minute call."


Read more: http://www.smh.com.au/digital-life/mobiles/cheap-ios-7-data-calls-a-threat-to-telcos-20130925-2ud2z....