One ring scam

Dear O2,

I just recieved a call from Mauritania(+222 38445850) and it rang for 7 seconds. I called back by mistake and there was a song playing in the phone. I thought I was traped in a scame. I am really afriad that I was charged for higly expensive bill. Therefore, I want to report this scam and ask if it is going to add more fees to my monthly rent since I have held up the phone for around 2 and half minites and I could not check the bill at the moment. Is there any security risk for my phone ? What shall I do to prevent potential costs? thanks.