Yealink SIP-T29G User Manual page 175

Advanced Phone Features
BLA number. BLA depends on support from a SIP server.
In the following figure, the first line is private and the second line is shared:
Any IP phone can be used to originate or receive calls on the bridged line. An incoming
call to the BLA number can be presented to multiple phones in the group simultaneously.
The incoming call can be answered on any IP phone of the group but not all.
You can share a BLA number on two or more phones. For example, phone A registers
the account 14019833020 and assigns BLA number, phone B registers the account
10462190020 and assigns BLA number, phone C registers the account 14755608020.
Phone A and phone B share the BLA number 14084588327.
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