Heh, I just upgrade my Flash from 2 to 4 channels not 30 minutes ago. Smooth as ice.
From what I gather it seems like the system is sending the internal users to the wrong extension. For example, my system the Flash is on 244, and the second port is 243. The DNs in sequence are: 244, 364, 243, 363. If you call any of the DNs other than 244, the display on the phone shows "(NAME) busy"
To tell if you're saturating your voice channels, just use a terminal to the serial port on the Flash (see the FAQ) Log in using "SYSMGR", and go to 4-Reports > 8-Alarms > 1-Alarm_Report. If the channels get saturated you'll see
07/02/05 20:44:39 pm.c: Port_Group_Busy received RetCode = 65535(ffff)
Hint: If you've got a 7310 w/BLF, program the Flash DNs as speed dial buttons for a quick, at-a-glance utilization indicator