Aastra 9143i Series Administrator's Manual page 526

BLA Subscription Period
The phones include a SIP BLA subscription period parameter that allows an
Administrator to set the amount of time, in seconds, of the BLA subscription
period.
Reference
For more information about setting the BLA Subscription Period, see the section,
"BLA Subscription Period"
Using a BLA Line on the IP Phone
If you have either a global or per-line BLA configuration, and you want to share a
call on the line with a BLA group, you need to press the Hold button before
sharing the call with the group.
For example, if line 1 is configured for BLA, and you pick up a call on line 1, you
must press the Hold button to share the call with the BLA group.
If you pick up a call on line 1 configured for BLA, and another call comes in on
line 2, you can pick up line 2 without putting line 1 on hold. The line 1 call will
be on hold automatically; however it is on hold locally only. The line 1 call
cannot be shared with the BLA group.
Note: The Hold button must be pressed for a call on a BLA line to be
shared with the BLA group.
on page 5-200.