Yealink SIP-T2 Series Administrator's Manual page 100

Administrator's Guide for SIP-T2 Series/T4 Series/T5 Series/CP920 IP Phones
Firmware Upgrade Configuration
Before upgrading firmware, you need to know the following:
Do not close and refresh the browser when the IP phone is upgrading firmware via web user interface.
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Do not unplug the network cables and power cables when the IP phone is upgrading firmware.
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The following table lists the parameters you can use to upgrade firmware.
static.firmware.url
Parameter
It configures the access URL of the firmware file.
Description Example:
static.firmware.url = http://192.168.1.20/44.84.0.10.rom
Permitted Values URL within 511 characters
Default Blank
Web UI Settings->Upgrade->Select And Upgrade Firmware
[1]
If you change this parameter, the IP phone will reboot to make the change take effect.
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[1]
<y0000000000xx>.cfg