T21 control send volume via webgui

[Yealink Support]

Can you try the syntax about sending volume for T2X to T21?

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#!version:1.0.0.1
##File header "#!version:1.0.0.1" can not be edited or deleted, and must be placed in the first line.##

#Configure the sending volume of Speaker, Handset and Headset. It ranges from 1 to 53, the default values are 25, 35, 29.
#Require reboot;
voice.handfree_send =
voice.handset_send =
voice.headset_send =
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