Well as it turned out - that wasn’t actually it. It’s something specific to the Cisco ISR4321.
I tried turning off OPTIONS and got the same problem. So then I thought heck with it, set the Cisco SIP server to port 5060, disabled the chan_sip trunks going to the Cisco 1760, and replaced them with chan_pjsip trunks with the Qualify Frequency set to 0. Those worked. Then for grins I set Qualify Frequency back to 60 - and, the trunks continued to work. With the benefit that now they show as “Available” in the Reports, Asterisk Info:
Endpoint: 5032313883P Not in use 0 of inf
Aor: 5032313883P 0
Contact: 5032313883P/sip:172.16.1.6 ee28828470 Avail 15.934
Transport: 0.0.0.0-udp udp 3 96 0.0.0.0:5060
Identify: 5032313883P/5032313883P
Match: 172.16.1.6/32
Endpoint: 5032355833P Not in use 0 of inf
Aor: 5032355833P 0
Contact: 5032355833P/sip:172.16.1.6 ee28828470 Avail 16.202
Transport: 0.0.0.0-udp udp 3 96 0.0.0.0:5060
Identify: 5032355833P/5032355833P
Match: 172.16.1.6/32
The pjsip trunks were created simply by Connectivity, Trunks, put in the trunk name, and channel 1 in general settings, and the SIP Server IP address in the pjsip general settings, and setting authentication and registration settings to None.
I then for fun went back and in the Trunk configuration pjsip settings general, set Authentication to Outbound and put in a username and secret, and then in the Cisco config, under sip-us, put in the same under an authentication directive. That worked fine as well.
So it’s definitely something in the IRS4321 “CUBE”
Now, I DID notice in the following post from 13 years ago:
That that configuration listed:
no mgcp package-capability res-package
no mgcp package-capability fxr-package
no mgcp timer receive-rtcp
no mgcp explicit hookstate
now, he was working with a 2800 not a ISR4321 - but for a while the 2800’s there had Cisco so-called “CUBE” functionality (Cisco Unified Border Element) AKA Session Border Controller, and turning the media gateway control protocol on might be interfering in some manner with using it for ordinary SIP. The ISR4321 was, after all, positioned by Cisco as their “primo” Session Border Controller entry to the market.
It should be noted that the ISR4321 End of Sale was in 2023 and End of Support from Cisco will be in 2028. Sometime during the lifespan of that device, Cisco modified the firmware so that newer firmwares for the device now require a yearly subscription fee. There’s a LOT of ISR4321’s out there which people are NOT firmware updating to the latest firmware (even if they are on support contract with Cisco) because that turns on the subscription “call home to mommie” feature, and thus the device will cease operating in 2028 (at least, as a SBC.) even if the owner is willing to pay the yearly fee.
Which is why the wise don’t use these (as well as the 2800/1760/etc. as Internet-touching devices.
I will continue testing the ISR4321 as there should be some way of getting this working.