Yealink SIP-T48S User Manual page 201

The following figure shows an example:
The Start REC soft key controls the recording function, and is available:
The free space of USB flash drive is sufficient

When there are one or more calls connected to your IP phone
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During an active call
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When calls are on hold or muted
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During an attended transfer
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During a conference call
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The Start REC soft key is not available when:
You place a new call
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No free space of USB flash drive
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The phone prompts you to answer an incoming call
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Call Recording
You can record active calls or conferences.
To record a call:
Tap the More soft key, and then tap the Start REC soft key during a call.
1.
Advanced Phone Features
191