Ultimate Volt is billed in two parts and that should have been explained when you changed over, as it's certainly made clear to new customers taking out any Volt bundle that there is a Virgin Media bill and separate O2 bill.
That contract you've posted is for Virgin Media services only - so the Mobile part is for Virgin Mobile. As Ultimate Volt doesn't come with Virgin Mobile then that's why it's not included
You should have received separate contract information from O2 for your SIM when you took it out.
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I work for Virgin Media - but all opinions posted here are my own