Yes your mum could speak to them but she would have to be with you when you make the call. They would ask you security questions etc and once you explained the issue, your Mum could then take over the conversation. You could also ask for her to be your representative.
Initially though YOU would have to speak to them first to ask for this arrangement. O2 need to ensure it's you in the first place. It's all a matter of security.
Hope that helps, best of luck and welcome to the forum.
Veritas Numquam Perit