OK, well you need to Print out your feature packages in LD 22 (PRT,PKG). Look for PKG 150 (DNXP) DN Expansion. This allows you to have DN's up to 7 digits.
Meridian mail doesn't care about the DN length. However, in any auto attendants you may have, you may want to adjust the MAX Digits option, but thats on an individual basis for each Auto Attendant.
Hope this helps...
Kris