When an firmware update is started you would be able to see that the DECT manager (with DECT base enabled) is already started but the DECT base stays disabled for some time, which can take up to 900 seconds.
The expected time for the whole system can be calculated: Upgrade time = Time for DM firmware update + (360 seconds + number of BSs * 10 seconds, limited to max timer = 900 seconds.)
The delay ensures a good and stable firmware update. Handsets will try to connect to active base stations when they are in range. During firmware update, base stations will start/restart, to avoid many handsets try to connect and cause high load which could cause instability, a delay is implemented. After the firmware update is ready the whole DECT system will start and handsets will have all base stations available to reconnect.