Point 2: Looking at the below trace, can't find anything wrong (except the 404 response for BYE at the end). Moreover, it looks to me that the INVITE gets positively completed, taking into account the 200 OK + MOH getting played. Yet, the mentioned error appears after 4-5 seconds, at the same moment the call gets removed from mohqcalls table .. as for the MOH, it keeps playing as expected.

Hints and comments are appreciated!

U 2017/06/14 14:56:28.132052 +0300 172.16.21.12:5060 -> 10.110.3.57:5060

INVITE sip:demo@10.110.3.57 SIP/2.0
Via: SIP/2.0/UDP 172.16.21.12:5060;rport;branch=z9hG4bKPjb1ac7743-b936-4074-9a5d-98e345a33f26
Max-Forwards: 70
From: <sip:6010@10.110.3.57>;tag=8bfd31ff-59ad-4efd-8ae5-47fde1862ef1
To: <sip:demo@10.110.3.57>
Contact: <sip:6010@172.16.21.12:5060>
Call-ID: 2116d060-8211-4f5c-beba-4a7d6c4fd08f
CSeq: 14866 INVITE
Subject: Phone call
Allow: SUBSCRIBE, NOTIFY, REFER, INVITE, ACK, BYE, CANCEL, UPDATE, INFO, OPTIONS, MESSAGE, PUBLISH
Supported: replaces
Content-Type: application/sdp
Content-Length:   399

v=0
o=yakama 3706430188 0 IN IP4 172.16.21.12
s=sflphone
c=IN IP4 172.16.21.12
t=0 0
m=audio 32298 RTP/AVP 0 3 8 9 110 111 112 101
a=rtpmap:0 PCMU/8000
a=rtpmap:3 GSM/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:9 G722/8000
a=rtpmap:110 speex/8000
a=rtpmap:111 speex/16000
a=rtpmap:112 speex/32000
a=sendrecv
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=rtcp:32299 IN IP4 172.16.21.12

U 2017/06/14 14:56:28.132412 +0300 10.110.3.57:5060 -> 172.16.21.12:5060

SIP/2.0 407 Proxy Authentication Required
Via: SIP/2.0/UDP 172.16.21.12:5060;rport=5060;branch=z9hG4bKPjb1ac7743-b936-4074-9a5d-98e345a33f26;received=172.16.21.12
From: <sip:6010@10.110.3.57>;tag=8bfd31ff-59ad-4efd-8ae5-47fde1862ef1
To: <sip:demo@10.110.3.57>;tag=20db48d68ad8cea70ceb54cf4988a90d.fe0d
Call-ID: 2116d060-8211-4f5c-beba-4a7d6c4fd08f
CSeq: 14866 INVITE
Proxy-Authenticate: Digest realm="10.110.3.57", nonce="WUElmFlBJGysi4cnHPxRxIhhn6VcSB1i"
Server: kamailio (4.2.3 (x86_64/linux))
Content-Length: 0


U 2017/06/14 14:56:28.142416 +0300 172.16.21.12:5060 -> 10.110.3.57:5060

ACK sip:demo@10.110.3.57 SIP/2.0
Via: SIP/2.0/UDP 172.16.21.12:5060;rport;branch=z9hG4bKPjb1ac7743-b936-4074-9a5d-98e345a33f26
Max-Forwards: 70
From: <sip:6010@10.110.3.57>;tag=8bfd31ff-59ad-4efd-8ae5-47fde1862ef1
To: <sip:demo@10.110.3.57>;tag=20db48d68ad8cea70ceb54cf4988a90d.fe0d
Call-ID: 2116d060-8211-4f5c-beba-4a7d6c4fd08f
CSeq: 14866 ACK
Content-Length:  0


U 2017/06/14 14:56:28.142768 +0300 172.16.21.12:5060 -> 10.110.3.57:5060

INVITE sip:demo@10.110.3.57 SIP/2.0
Via: SIP/2.0/UDP 172.16.21.12:5060;rport;branch=z9hG4bKPj63038e66-0dc3-4cce-8e07-04549f40d72b
Max-Forwards: 70
From: <sip:6010@10.110.3.57>;tag=8bfd31ff-59ad-4efd-8ae5-47fde1862ef1
To: <sip:demo@10.110.3.57>
Contact: <sip:6010@172.16.21.12:5060>
Call-ID: 2116d060-8211-4f5c-beba-4a7d6c4fd08f
CSeq: 14867 INVITE
Subject: Phone call
Allow: SUBSCRIBE, NOTIFY, REFER, INVITE, ACK, BYE, CANCEL, UPDATE, INFO, OPTIONS, MESSAGE, PUBLISH
Supported: replaces
Proxy-Authorization: Digest username="6010", realm="10.110.3.57", nonce="WUElmFlBJGysi4cnHPxRxIhhn6VcSB1i", uri="sip:demo@10.110.3.57", response="cf367b584f49117c1cf2d3a20ef629bd"
Content-Type: application/sdp
Content-Length:   399

v=0
o=yakama 3706430188 0 IN IP4 172.16.21.12
s=sflphone
c=IN IP4 172.16.21.12
t=0 0
m=audio 32298 RTP/AVP 0 3 8 9 110 111 112 101
a=rtpmap:0 PCMU/8000
a=rtpmap:3 GSM/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:9 G722/8000
a=rtpmap:110 speex/8000
a=rtpmap:111 speex/16000
a=rtpmap:112 speex/32000
a=sendrecv
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=rtcp:32299 IN IP4 172.16.21.12

U 2017/06/14 14:56:28.148654 +0300 10.110.3.57:5060 -> 172.16.21.12:5060

SIP/2.0 180 Ringing
Via: SIP/2.0/UDP 172.16.21.12:5060;rport=5060;branch=z9hG4bKPj63038e66-0dc3-4cce-8e07-04549f40d72b;received=172.16.21.12
From: <sip:6010@10.110.3.57>;tag=8bfd31ff-59ad-4efd-8ae5-47fde1862ef1
To: <sip:demo@10.110.3.57>;tag=2f2adbacc4317338a374a6531765d255-9497
Call-ID: 2116d060-8211-4f5c-beba-4a7d6c4fd08f
CSeq: 14867 INVITE
Contact: <sip:demo@10.110.3.57:5060>
Server: kamailio (4.2.3 (x86_64/linux))
Content-Length: 0


U 2017/06/14 14:56:28.149767 +0300 10.110.3.57:5060 -> 172.16.21.12:5060

SIP/2.0 200 OK
Via: SIP/2.0/UDP 172.16.21.12:5060;rport=5060;branch=z9hG4bKPj63038e66-0dc3-4cce-8e07-04549f40d72b;received=172.16.21.12
From: <sip:6010@10.110.3.57>;tag=8bfd31ff-59ad-4efd-8ae5-47fde1862ef1
To: <sip:demo@10.110.3.57>;tag=2f2adbacc4317338a374a6531765d255-9497
Call-ID: 2116d060-8211-4f5c-beba-4a7d6c4fd08f
CSeq: 14867 INVITE
Contact: <sip:demo@10.110.3.57:5060>
Allow: INVITE, ACK, BYE, CANCEL, NOTIFY, PRACK
Supported: 100rel
Accept-Language: en
Content-Type: application/sdp
User-Agent: Kamailio MOH Queue v1.0
Server: kamailio (4.2.3 (x86_64/linux))
Content-Length: 166

v=0
o=- 1 1 IN IP4 10.110.3.57
s=Kamailio MOH Queue v1.0
c=IN IP4 10.110.3.57
t=0 0
a=sendrecv
m=audio 49464 RTP/AVP 3
a=rtpmap:3 GSM/8000
a=nortpproxy:yes

U 2017/06/14 14:56:28.165176 +0300 172.16.21.12:5060 -> 10.110.3.57:5060

ACK sip:demo@10.110.3.57:5060 SIP/2.0
Via: SIP/2.0/UDP 172.16.21.12:5060;rport;branch=z9hG4bKPj69bbf40b-cc63-4699-bc05-fb41e210ac12
Max-Forwards: 70
From: <sip:6010@10.110.3.57>;tag=8bfd31ff-59ad-4efd-8ae5-47fde1862ef1
To: <sip:demo@10.110.3.57>;tag=2f2adbacc4317338a374a6531765d255-9497
Call-ID: 2116d060-8211-4f5c-beba-4a7d6c4fd08f
CSeq: 14867 ACK
Content-Length:  0


U 2017/06/14 14:56:34.592956 +0300 172.16.21.12:5060 -> 10.110.3.57:5060

BYE sip:demo@10.110.3.57:5060 SIP/2.0
Via: SIP/2.0/UDP 172.16.21.12:5060;rport;branch=z9hG4bKPjd58e78ac-48c1-440c-833c-7d6b6c91038b
Max-Forwards: 70
From: <sip:6010@10.110.3.57>;tag=8bfd31ff-59ad-4efd-8ae5-47fde1862ef1
To: <sip:demo@10.110.3.57>;tag=2f2adbacc4317338a374a6531765d255-9497
Call-ID: 2116d060-8211-4f5c-beba-4a7d6c4fd08f
CSeq: 14868 BYE
Contact: <sip:6010@172.16.21.12:5060>
Content-Length:  0


U 2017/06/14 14:56:34.593103 +0300 10.110.3.57:5060 -> 172.16.21.12:5060

SIP/2.0 404 Not here
Via: SIP/2.0/UDP 172.16.21.12:5060;rport=5060;branch=z9hG4bKPjd58e78ac-48c1-440c-833c-7d6b6c91038b;received=172.16.21.12
From: <sip:6010@10.110.3.57>;tag=8bfd31ff-59ad-4efd-8ae5-47fde1862ef1
To: <sip:demo@10.110.3.57>;tag=2f2adbacc4317338a374a6531765d255-9497
Call-ID: 2116d060-8211-4f5c-beba-4a7d6c4fd08f
CSeq: 14868 BYE
Server: kamailio (4.2.3 (x86_64/linux))
Content-Length: 0