Yealink SIP-T4X Provisioning Manual page 58

Yealink SIP-T4X IP Phones Auto Provisioning Guide
You can add the option 66 via DHCP Turbo. The following shows the detailed processes.
1. Click Named Policies-->Global, right click the blank area on the right of the main
page and then select New Option.
2. Select TFTP Options from the pull-down list of Filter.
3. Scroll down and double click MS option 66.
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