Yealink SIP-T42S User Manual page 185

Click on Advanced, and then select Draft BLA from the pull-down list of Shared Line.
2.
Enter the desired number in the BLA Number field.
3.
Click Confirm to accept the change.
4.
To register an account and configure BLA feature on phone B via web user interface:
Register the account 10462190020.
1.
Advanced Phone Features
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