Don't know how it works but here we go...
Can it happen that during the transition from EE to VF a phone(Virgin sim only) will connect to the EE signal whilst VF is available at the local mast, then, after inverting the new sim card , it not only connects to the VF signal but can connect to the EE signal when near a mast that does not transmit the Virgin VF signal?
Whilst you're there, could my phone do the EE-VF transition without a sim change, without me knowing?
Have a very poor Virgin-EE signal, can't wait for the jump to VF.