You could also do a find/replace (or, rather two of them if needed) to remove the leading character.
So for your "D" phone number example, you would do a F/R after the merge like so:
D <FEFF>\r
If I remembered the syntax correctly, anyway. It finds the literal 'D' followed by a space, followed by an empty merge field marker, followed by a return. Replace would be with nothing.
Else consider using a merge product that has the conditional capabilities. Em Software makes one such plug-in. I use Xdata for QuarkXpress a good bit, their InDesign counterpart is called InData.
Mike