Could be a phone fault, could be that they connect to different networks.
VM are a "virtual" mobile network and like all virtual networks have to use one of the big four "physical" networks (EE, O2, Vodafone, Three). VM are in the process of changing their mobile customers from EE to Vodafone's network, so it is quite possible that the two phones both say Virgin Media, but are connected to completely different networks.
If the poor reception is down to the EE to Vodafone move, then your only fix is to port the number to a network that gives you better coverage. And it is possible that the 12 will be ported in due course, and coverage will deteriorate, although you should bear in mind I'm not in your house, I can't do the sort of checks that would confirm or reject my suspicions on the network change being the cause of your problem.
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