Can you try it on a neighbour's line? If it works OK elsewhere then it's a line issue. If it doesn't then it's the box at fault.
Of course, it may be that the BT line meets its spec but the box is relying on some undocumented feature that no longer exists on your line. There used to be a lot of that in telecoms, things were done a particular way 'because it works'. Even if BT can be shown to be in violation of their specs you can still be out of luck (been there done that, broke the exchange so they had to fix it...), and if they conform to the spec then you're definitely out of luck.
What changes (if any) were made to your in-house wiring? Have you tried unplugging any other phones on the system, or trying a different phone with the box? It could be something like the line polarity has changed (the pair of wires got swapped somewhere, not normally a problem) and something with a fault that didn't show up one way is now causing an issue.