Format of properly formed 200 ACK response in VOIP communications - twilio

Question

We have a Grandstream UCM6204 PBX connecting to a registered SIP elastic trunk through Twilio. Our trunk originating calls drop after about 60 seconds. Twilio is blaming Grandstream but Grandstream is blaming Twilio. Each is saying the other is not properly formatting or not reading the packet correctly. Pcap log with single 200 ACK packet in question exported.
According to Twilio, the packet should have an acknowledge something like this:
ACK sip:172.18.23.153:5060 SIP/2.0
Grandstream says they (their unit) won't send it to the private IP address and should be sent to the record-route IP address where it will be forwarded to the private IP based on header info. I'm too much of a newbie here to know what's right and what's wrong so any suggestions would be great.
No. Time Source Destination Protocol Length Info
3 0.119432 96.10.11.12 54.172.60.2 SIP/SDP 1217 Status: 200 OK |
Frame 3: 1217 bytes on wire (9736 bits), 1217 bytes captured (9736 bits)
Encapsulation type: Linux cooked-mode capture (25)
Arrival Time: Dec 19, 2018 13:51:32.749232000 Eastern Standard Time
[Time shift for this packet: 0.000000000 seconds]
Epoch Time: 1545245492.749232000 seconds
[Time delta from previous captured frame: 0.050481000 seconds]
[Time delta from previous displayed frame: 0.050481000 seconds]
[Time since reference or first frame: 0.119432000 seconds]
Frame Number: 3
Frame Length: 1217 bytes (9736 bits)
Capture Length: 1217 bytes (9736 bits)
[Frame is marked: False]
[Frame is ignored: False]
[Protocols in frame: sll:ethertype:ip:udp:sip:sdp]
[Coloring Rule Name: UDP]
[Coloring Rule String: udp]
Linux cooked capture
Packet type: Unicast to us (0)
Link-layer address type: 1
Link-layer address length: 6
Source: 0e:d7:c3:5c:55:24 (0e:d7:c3:5c:55:24)
Unused: 0000
Protocol: IPv4 (0x0800)
Internet Protocol Version 4, Src: 96.10.11.12, Dst: 54.172.60.2
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
0000 00.. = Differentiated Services Codepoint: Default (0)
.... ..00 = Explicit Congestion Notification: Not ECN-Capable Transport (0)
Total Length: 1201
Identification: 0x0000 (0)
Flags: 0x4000, Don't fragment
0... .... .... .... = Reserved bit: Not set
.1.. .... .... .... = Don't fragment: Set
..0. .... .... .... = More fragments: Not set
...0 0000 0000 0000 = Fragment offset: 0
Time to live: 39
Protocol: UDP (17)
Header checksum: 0x3257 [validation disabled]
[Header checksum status: Unverified]
Source: 96.10.11.12
Destination: 54.172.60.2
User Datagram Protocol, Src Port: 5060, Dst Port: 5060
Source Port: 5060
Destination Port: 5060
Length: 1181
Checksum: 0xfa57 [unverified]
[Checksum Status: Unverified]
[Stream index: 0]
Session Initiation Protocol (200)
Status-Line: SIP/2.0 200 OK
Status-Code: 200
[Resent Packet: False]
[Request Frame: 1]
[Response Time (ms): 119]
Message Header
Via: SIP/2.0/UDP 54.172.60.2:5060;rport=5060;received=54.172.60.2;branch=z9hG4bK0d03.870e90c7.0
Transport: UDP
Sent-by Address: 54.172.60.2
Sent-by port: 5060
RPort: 5060
Received: 54.172.60.2
Branch: z9hG4bK0d03.870e90c7.0
Via: SIP/2.0/UDP 172.18.15.132:5060;rport=5060;received=172.18.15.132;branch=z9hG4bK5d21b0af-3104-4e32-96a7-7e89edd39a93_6772d868_310-9499633517006747707
Transport: UDP
Sent-by Address: 172.18.15.132
Sent-by port: 5060
RPort: 5060
Received: 172.18.15.132
Branch: z9hG4bK5d21b0af-3104-4e32-96a7-7e89edd39a93_6772d868_310-9499633517006747707
Record-Route: <sip:54.172.60.2:5060;lr;ftag=90419967_6772d868_5d21b0af-3104-4e32-96a7-7e89edd39a93>
Record-Route URI: sip:54.172.60.2:5060;lr;ftag=90419967_6772d868_5d21b0af-3104-4e32-96a7-7e89edd39a93
Record-Route Host Part: 54.172.60.2
Record-Route Host Port: 5060
Record-Route URI parameter: lr
Record-Route URI parameter: ftag=90419967_6772d868_5d21b0af-3104-4e32-96a7-7e89edd39a93
Call-ID: 937861cfda42d874af27d391663108b1#0.0.0.0
From: <sip:[phone]#somebody.pstn.twilio.com>;tag=90419967_6772d868_5d21b0af-3104-4e32-96a7-7e89edd39a93
SIP from address: sip:[phone]#somebody.pstn.twilio.com
SIP from address User Part: [phone]
E.164 number (MSISDN): 12485551212
Country Code: Americas (1)
SIP from address Host Part: somebody.pstn.twilio.com
SIP from tag: 90419967_6772d868_5d21b0af-3104-4e32-96a7-7e89edd39a93
To: <sip:[phone]#96.10.11.12>;tag=68625bc3-5cb5-4bd4-a255-ce12744a3514
SIP to address: sip:[phone]#96.10.11.12
SIP to address User Part: [phone]
E.164 number (MSISDN): 12486661212
Country Code: Americas (1)
SIP to address Host Part: 96.10.11.12
SIP to tag: 68625bc3-5cb5-4bd4-a255-ce12744a3514
CSeq: 28344 INVITE
Sequence Number: 28344
Method: INVITE
Server: Grandstream UCM6204V1.5A 1.0.18.12
Contact: <sip:[phone]#192.168.4.10:5060>
Contact URI: sip:[phone]#192.168.4.10:5060
Contact URI User Part: [phone]
E.164 number (MSISDN): 12485551212
Country Code: Americas (1)
Contact URI Host Part: 192.168.4.10
Contact URI Host Port: 5060
Allow: OPTIONS, INFO, SUBSCRIBE, NOTIFY, PUBLISH, INVITE, ACK, BYE, CANCEL, UPDATE, PRACK, REFER, MESSAGE, REGISTER
Supported: 100rel, timer, replaces, norefersub
Content-Type: application/sdp
Content-Length: 237
Message Body
Session Description Protocol
Session Description Protocol Version (v): 0
Owner/Creator, Session Id (o): - 1796373137 1796373139 IN IP4 192.168.4.10
Owner Username: -
Session ID: 1796373137
Session Version: 1796373139
Owner Network Type: IN
Owner Address Type: IP4
Owner Address: 192.168.4.10
Session Name (s): Asterisk
Connection Information (c): IN IP4 192.168.4.10
Connection Network Type: IN
Connection Address Type: IP4
Connection Address: 192.168.4.10
Time Description, active time (t): 0 0
Session Start Time: 0
Session Stop Time: 0
Media Description, name and address (m): audio 13894 RTP/AVP 0 101
Media Type: audio
Media Port: 13894
Media Protocol: RTP/AVP
Media Format: ITU-T G.711 PCMU
Media Format: DynamicRTP-Type-101
Media Attribute (a): rtpmap:0 PCMU/8000
Media Attribute Fieldname: rtpmap
Media Format: 0
MIME Type: PCMU
Sample Rate: 8000
Media Attribute (a): rtpmap:101 telephone-event/8000
Media Attribute Fieldname: rtpmap
Media Format: 101
MIME Type: telephone-event
Sample Rate: 8000
Media Attribute (a): fmtp:101 0-16
Media Attribute Fieldname: fmtp
Media Format: 101 [telephone-event]
Media format specific parameters: 0-16
Media Attribute (a): ptime:20
Media Attribute Fieldname: ptime
Media Attribute Value: 20
Media Attribute (a): maxptime:150
Media Attribute Fieldname: maxptime
Media Attribute Value: 150
Media Attribute (a): sendrecv