Why is the SIP ACK sent from our PBX not received by Twilio?

Question

Our outbound calls through Twilio are all being dropped after 32 seconds (inbound calls are fine). The error we receive from Twilio is Error 32022 "Ack not received from your SIP endpoint." I have been going through tutorials and the RFC but I must really be missing something.

The calls are encrypted TLS. SIP ALG has been disabled in our gateway router and our Grandstream PBX is configured for NAT.

Any help is appreciated!

Here is the 200 OK and the ACK that never arrives:

<--- Received SIP response (1177 bytes) from TLS:54.172.60.2:5061 ---> 
SIP/2.0 200 OK
CSeq: 2218 INVITE
Call-ID: b9e1de31-ce44-4d34-b63b-4c8636e151bf
From: <sip:+13049487110@XXXX>;tag=814fce4e-a1dc-49c7-a917-59496ad182a7
To: <sip:+15132607811@xxxx.pstn.twilio.com>;tag=84858964_c3356d0b_d52c7a82-314f-4be6-88f4-e73110bdbdd8
Via: SIP/2.0/TLS 74.195.14.236:5061;received=74.195.14.236;rport=52191;branch=z9hG4bKPj42ac7abe-f4b7-40dc-a314-8c6a90584481;alias
Record-Route: <sip:54.172.60.2:5060;r2=on;lr;twnat=sip:74.195.14.236:52191>
Record-Route: <sip:54.172.60.2:5061;transport=tls;r2=on;lr;twnat=sip:74.195.14.236:52191>
Server: Twilio
Contact: <sip:172.25.75.195:5060>
Allow: INVITE,ACK,CANCEL,OPTIONS,BYE,REFER,NOTIFY
Content-Type: application/sdp
X-Twilio-CallSid: CAb39a1e013e6ad6c03bd5bd2827612646
X-Twilio-TlsPolicy: TLSv1.0+
Content-Length: 362

v=0
o=root 1866335776 1866335776 IN IP4 172.   ...


<--- Transmitting SIP request (1010 bytes) to TLS:54.172.60.2:5061 ---> 
ACK sip:54.172.60.2:5061;transport=TLS SIP/2.0
Via: SIP/2.0/TLS 74.195.14.236:5061;rport;branch=z9hG4bKPj483e2dff-e510-4908-89ee-25ec200a4072;alias
From: <sip:+13049487110@XXXX>;tag=814fce4e-a1dc-49c7-a917-59496ad182a7
To: <sip:+15132607811@xxxx.pstn.twilio.com>;tag=84858964_c3356d0b_d52c7a82-314f-4be6-88f4-e73110bdbdd8
Call-ID: b9e1de31-ce44-4d34-b63b-4c8636e151bf
CSeq: 2218 ACK
Route: <sip:54.172.60.2:5061;transport=tls;lr;r2=on;twnat=sip:74.195.14.236:52191>
Route: <sip:54.172.60.2:5060;lr;r2=on;twnat=sip:74.195.14.236:52191>
...

Answer 1

The ACK request s addressed incorrectly: per 3261 with loose routing it should be addressed to the Contact URI, but sent to the topmost Route (built from the bottommost Record-Route, the route-set in your ACK appears to be correct). Correct the request URI for the ACK, that should help.