£70 for 30mins calls starts with 0300

[Eddy2]

I used the app Rebtel to call my friend outside the UK and the number starts with 0300.  I suddenly found that there was a £70 local call fees in my bill.  Could anyone help me what is it? Why would O2 charge me that? I have not encountered any charges when I used WhatsApp to call my friends outside the UK. 

[MI5]

@Eddy2 

You weren't billed by O2 directly. The charge is generated by Rebtel - It's not a free service. O2 have passed on the charges from Rebtel, just in the same way as if you used your mobile to pay for something.

[Eddy2]

I paid Rebtel separately.  But O2 charged me £2.5 per min for making Rebtel calls starting with “0300”