Hello! Please tell me how you sorted this as I'm having the same problem! I had a sim as part of the oomph package that never came out of the packet and when I got put on volt I got another sim with a separate monthly charge directlyfrom o2. As the oomph package payment covered the price of the sim when went onto volt the sim wasn't being paid anymore so o2 are trying to bill me .
Welcome to the Community Forums and thanks for the post.
Sorry to hear about the confusion, with what happens with Volt packages the SIM is paid directly through O2, however what would have been done is a discount applied to your Virgin Media side to offset this charge, this would be part of the price agreed, if you look at your contract you will have one provided by Virgin Media stating a set price of X amount, then a separate one from O2.