14.4.11.4.2. Number Routing for 5-digit On-Base-Numbers 
If Base Numbers don't comply with the GSCR (see Section
174.1, "Interswitch and Intraswitch Dialing"),
phone numbers can create a conflict with DSN numbers (see example below).
This document
also describes a possible workaround for US military bases with a conflicting
dial plan.