Yealink SIP-T21 - Manual (Page 6)

User Guide for the SIP-T21P/SIP-T21 IP Phone
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Anonymous Call on page 93
Documentations of the SIP-T21 IP phone have also been added.
Major updates have occurred to the following sections:
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LED Instructions on page 4
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Anonymous Call on page 93
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Troubleshooting on page 117