I rang a South African mobile using skype credit, the rate on the website is € 0.217 inc VAT.
I made a call for 14 mins 30 secs, it cost me €5.18,
A 15 minute call to a South African cellphone should have cost Euro 4.05 including the connection fee.
You appear to have been charged for a 24 minute call.
Please check at your Account Page - ''Call History'' to check the call timing and advise.