Is the issue with signal or the operation of the phone. What tariff are you on and which country are you trying to call etc.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
Sounds like it may be the operation of the phone? @adrianasemushin If you tell us which phone you have then we would be happy to help you. There are symbols on your phone to enable mobile data. They are two arrows one pointing up and one pointing down only phone. I press this and my phone automatically connects to the mobile data. Or how to switch on your phone. Usually buttons on the side of your phone press and phone opens to receive the network. Bye for now TallTrees