Assuming that the handset isn't faulty, most probable explanation is that VM are transferring customers from their old network operator (EE) to a new one (Vodafone), and therefore although both phones say "Virgin Media" they could be on different networks, with local coverage variation.
If the phone without reception is on the EE network, then forum staff may be able to have the transfer to Vodafone brought forward. If the phone without reception is on Vodafone, then nothing can be done, and in a matter of months you may have two phones with no reception when that one is moved on to the Vodafone network.
Do you know from visitors how reception in the house is for EE or Vodafone signals?