So, I went for dinner and one of the BLFs started working while I was away from my desk... I left sngrep running and it captured something puzzling....
------ BEFORE - NOT WORKING ---------

2021/10/02 19:05:43.842970 CLIENT_IP:55318 -> PUBLIC_IP:PORT
SUBSCRIBE sip:301@domain.net:PORT SIP/2.0
Via: SIP/2.0/UDP 10.10.10.62:55318;branch=z9hG4bK89955442;rport
From: <sip:305@domain.net:PORT>;tag=1703808353
To: <sip:301@domain.net:PORT>
Call-ID: 2104033891-55318-6@BA.CEG.HG.GC
CSeq: 86910 SUBSCRIBE
Contact: <sip:305@10.10.10.62:55318>
X-Grandstream-PBX: true
Max-Forwards: 70
User-Agent: Grandstream GXP1782 1.0.1.116
Expires: 60
Supported: replaces, path, timer, eventlist
Event: dialog
Accept: application/dialog-info+xml,multipart/related,application/rlmi+xml
Accept-Encoding: zlib+base64
Allow: INVITE, ACK, OPTIONS, CANCEL, BYE, SUBSCRIBE, NOTIFY, INFO, REFER, UPDATE, MESSAGE
Content-Length: 0

2021/10/02 19:05:43.843367 PUBLIC_IP:PORT -> CLIENT_IP:55318
SIP/2.0 202 OK
Via: SIP/2.0/UDP 10.10.10.62:55318;branch=z9hG4bK89955442;rport=55318;received=CLIENT_IP
From: <sip:305@domain.net:PORT>;tag=1703808353
To: <sip:301@domain.net:PORT>;tag=5e1ee053cf7168b9d3ee9c05740f837c-270107a6
Call-ID: 2104033891-55318-6@BA.CEG.HG.GC
CSeq: 86910 SUBSCRIBE
Expires: 60
Contact: <sip:PUBLIC_IP:PORT>
Server: kamailio (5.4.4 (x86_64/linux))
Content-Length: 0

2021/10/02 19:05:43.844037 PUBLIC_IP:PORT -> CLIENT_IP:55318
NOTIFY sip:305@10.10.10.62:55318 SIP/2.0
Via: SIP/2.0/UDP PUBLIC_IP:PORT;branch=z9hG4bKffd9.6b44c1b2000000000000000000000000.0
To: <sip:305@domain.net>;tag=1703808353
From: <sip:301@domain.net>;tag=5e1ee053cf7168b9d3ee9c05740f837c-270107a6
CSeq: 2 NOTIFY
Call-ID: 2104033891-55318-6@BA.CEG.HG.GC
Content-Length: 273
User-Agent: kamailio (5.4.4 (x86_64/linux))
Max-Forwards: 70
Event: dialog
Contact: <sip:PUBLIC_IP:PORT>
Subscription-State: active;expires=60
Content-Type: application/dialog-info+xml

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info" version="1" state="full" entity="sip:301@domain.net">
  <dialog id="615293b33c62dec073e05d9421e9f48b" direction="recipient"> <state>terminated</state> </dialog>
</dialog-info>
------ BEFORE - NOT WORKING ---------

------ AFTER - WORKING ---------

2021/10/02 19:06:04.291019 CLIENT_IP:55318 -> PUBLIC_IP:PORT
SUBSCRIBE sip:301@domain.net:PORT SIP/2.0
Via: SIP/2.0/UDP 10.10.10.62:55318;branch=z9hG4bK1198306856;rport
From: <sip:305@domain.net:PORT>;tag=998538926
To: <sip:301@domain.net:PORT>
Call-ID: 2104033891-55318-6@BA.CEG.HG.GC
CSeq: 86920 SUBSCRIBE
Contact: <sip:305@10.10.10.62:55318>
X-Grandstream-PBX: true
Max-Forwards: 70
User-Agent: Grandstream GXP1782 1.0.1.116
Expires: 60
Supported: replaces, path, timer, eventlist
Event: dialog
Accept: application/dialog-info+xml,multipart/related,application/rlmi+xml
Accept-Encoding: zlib+base64
Allow: INVITE, ACK, OPTIONS, CANCEL, BYE, SUBSCRIBE, NOTIFY, INFO, REFER, UPDATE, MESSAGE
Content-Length: 0

2021/10/02 19:06:04.291688 PUBLIC_IP:PORT -> CLIENT_IP:55318
SIP/2.0 202 OK
Via: SIP/2.0/UDP 10.10.10.62:55318;branch=z9hG4bK1198306856;rport=55318;received=CLIENT_IP
From: <sip:305@domain.net:PORT>;tag=998538926
To: <sip:301@domain.net:PORT>;tag=5e1ee053cf7168b9d3ee9c05740f837c-270107a6
Call-ID: 2104033891-55318-6@BA.CEG.HG.GC
CSeq: 86920 SUBSCRIBE
Expires: 60
Contact: <sip:PUBLIC_IP:PORT>
Server: kamailio (5.4.4 (x86_64/linux))
Content-Length: 0

2021/10/02 19:06:04.292671 PUBLIC_IP:PORT -> CLIENT_IP:55318
NOTIFY sip:305@10.10.10.62:55318 SIP/2.0
Via: SIP/2.0/UDP PUBLIC_IP:PORT;branch=z9hG4bKffd9.40e91c22000000000000000000000000.0
To: <sip:305@domain.net>;tag=998538926
From: <sip:301@domain.net>;tag=5e1ee053cf7168b9d3ee9c05740f837c-270107a6
CSeq: 2 NOTIFY
Call-ID: 2104033891-55318-6@BA.CEG.HG.GC
Content-Length: 273
User-Agent: kamailio (5.4.4 (x86_64/linux))
Max-Forwards: 70
Event: dialog
Contact: <sip:PUBLIC_IP:PORT>
Subscription-State: active;expires=60
Content-Type: application/dialog-info+xml

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info" version="1" state="full" entity="sip:301@domain.net">
  <dialog id="615293b33c62dec073e05d9421e9f48b" direction="recipient"> <state>terminated</state> </dialog>
</dialog-info>
------ AFTER - WORKING ---------

The careful eye will notice NOTHING CHANGED.....

Almost nothing... Of course the branch and (from/from/to) tags are different BUT THATS IT...
So, not working, go for food, working... I changed nothing.
I thought it was the never change version attribute in the dialog-info but, maybe not.
Has anyone seen this ? If so, how did you get past it ?
In case you're new to this thread, these are Grandstream GXP1782 desk phones.
When I change the DNS A record for the host and point it to a FreeSWITCH system, the BLFs are fine. It's just when they reach Kamailio 5.4.4 that they don't sync up properly.

Any pointers are welcome...
Jerry