Yealink SIP-T56A User Manual page 211

Do one of the following:
4.
Long tap one of the pictures.
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Tap on other pictures to select multiple pictures.
Tap and then tap Select item.
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Tap the desired picture(s) to select multiple pictures.
You can also tap n items selected (n represents the number of pictures you selected)
and then tap Select all to select all pictures.
System Applications
211