If, by green screen, you mean the "DVR has detected an error" message, then you're correct to believe this will end with your box being replaced.
Ordinarily, there's 2 ways to report faults, depending how urgent they are and how quickly you want a response:
1: Call in - 150 (from a VM phone) or 0345 4541111 (from another phone) and choose the "I have a fault" options
2: Wait on here for VM staff to respond, this can take a couple of days.
Out of interest, what's the issue you're having with calling in?
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