You've mentioned having 2 boxes and only one is affected, so that suggests the fault is either with the box or your internal cabling. After you return from holiday, could you try swapping the 2 boxes around and see if the fault follows the move? If it does, that'll show the box as faulty. If not, then you'll have a cabling fault. Knowing this, will greatly expedite whatever fix is necessary.
Staff will generally take a couple of days to respond on here, but calls to VM's 0345 number are charged at a landline rate - so whatever your mobile costs to call a UK landline, will be the same charge to any UK 03xx number.
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