User Guide: http://www.grandstream.com/sites/default/files/Resources/DP750_DP720_User_Guide.pdf

Press and hold the Radio/Page button for 7 seconds until the Radio icon starts blinking Or in the web UI, press Subscribe icon.

Press Subscribe softkey, if available on the main screen, or access Menu → Registration → Register while the DP750 Radio icon is blinking

The Subscribe softkey appears only if DP720 is not registered to any DP750 base station.

See the User Guide for battery information.

A good option:

Panasonic BK-4MCCA8BA eneloop AAA 2100 Cycle Ni-MH Pre-Charged Rechargeable Batteries

You should probably place all the handsets in their charging cradles before starting the update.

1. Log in to your DP750 base station using the web interface
   • The default credentials are admin / admin
2. Maintenance → Firmware Upgrade →
   1. Firmware Server Path → firmware.grandstream.com
   2. Handset Firmware Automatic Upgrade → Yes
3. Click Save and Apply
4. Reboot the base station

After the base reboots and upgrades to the latest version, the handsets will commence updating one at a time.

Be patient!