Well, after LOTS of trial and error, I think I have nailed it. Turns out Spectrum couldn’t give me enough information. They insisted it was a SIP trunk, using CHAN-SIP. Some furious googling, and I find another link from here:
He was trying to do something else, but what helped was the fact he used PJSIP. I redid everything from scratch and created the trunk connection to Spectrum using PJSIP, and it’s working. Spectrum was really less than helpful, but I kind of figured that would be the case.
If anyone comes across this thread looking for the same information, just drop me a PM and I will share the setup.