为何PBX发送的SIP ACK未被Twilio接收，外呼32秒后中断？

所有通过Twilio发起的外呼通话都会在32秒后中断，但呼入通话完全正常。收到Twilio返回的错误码32022，提示内容为：“未从您的SIP端点收到ACK”。

已完成的排查操作：

• 通话采用TLS加密方式
• 网关路由器已禁用SIP ALG
• Grandstream PBX已配置NAT

已查阅相关教程及RFC文档，但仍未定位问题原因，恳请各位提供技术帮助！

以下是收到的200 OK响应及未送达的ACK请求报文：

<--- 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>
...

内容的提问来源于stack exchange，提问作者bruce57