Charged extra

Hello , 

I had purchased O2 Homespot small (19.98 euros) for young people. But today I get a bill saying I have to pay 28.64 for one month. I dont understand why I am charged extra. And I have not made any phone calls as I have inserted the SIM card in the Homespot device. So technically it is impossible for me to make phone calls.

Kindly help me with this problem.