Hello!
I was testing the chan_pjsip driver this weekend, because I like the idea of sharing one extension on multiple endpoints.
When I was using chan_sip, I had created some BLFs for other extensions on my Yealink T38G. They worked fine. After I had changed to chan_pjsip, the BLFs stayed dark. After an hour of anger, I figured out, that BLFs start working again, when you set a presence state in the UCP. Then all ist fine: The BLFs light green, when the extension is not in use, blink, when the extension is ringing and red, when the extension is in use.
So, I see the benefits of coupling the “device”-presence to a “user”-presence. A user can log in to his UCP and set, let’s say, DND and this is reflected to all BLFs that point to his account.
But: At first, I don’t need that. I just want to see, if someone is on the phone or of the extension is ringing. Like it did with chan_sip.
Then, there is a problem: A user with multiple extensions would have to set his presence state in every single UCP, as it is not allowed to link one user to multiple phones.
Also, as administrator, you have to login to every UCP and set “available” as presence state. When registering multiple extensions, this might be quite a work.
Do you encounter the same behavior? Is it a bug, or is it a feature? 
Oh, nearly forgotten: I am using a FreePBX Distro, 10.13.66-5
Kind regards!
Andy