Akuvox E21 Series - Firmware and Provisioning; Firmware Upgrade

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4.11. Upgrade
4.11.1. Upgrade-Basic
Go to Upgrade->Basic, user can upgrade firmware; Reset to
factory setting and reboot.
4.11.2. Upgrade-Advanced
4.11.2.1. PNP
Plug and Play, once PNP is enabled, the phone will send SIP
subscription message to PNP server automatically to get
Auto Provisioning server’s address.
By default, this SIP message is sent to multicast address
224.0.1.75(PNP server address by standard).