Yealink SIP-T21 - Manual (Page 124)

User Guide for the SIP-T21P/SIP-T21 IP Phone
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5. Enter the period time in the MWI Subscription Period (Seconds) field.
6. Click Confirm to accept the change.
The IP phone will subscribe to the account number for MWI service by default.
To enable Subscribe MWI to Voice Mail via web user interface:
1. Click on Account.
2. Select the desired account from the pull-down list of Account.
3. Click on Advanced.
4. Select Enabled from the pull-down list of Subscribe MWI To Voice Mail.
5. Enter the desired voice mail number in the Voice Mail field.