Do you see the SIP.JS extension in FreeSwitch if you run: show registrations
Also I wouldn't expect to see a channel of type "[error]" in the log.
2023-11-28 14:04:42.951275 97.17% [NOTICE] switch_ivr_originate.c:3049 Cannot create outgoing channel of type [error] cause: [USER_NOT_REGISTERED]
My thought is NAT issue, though I'd have thought Sangoma Talk would use a STUN server to handle that.
The fact the phone keeps sending the REGISTER over and over, suggests it's not receiving the 401 response. I'd start by double checking the Telstra 4G IP address,
then making sure the REGISTER has the correct IP in the Contact header.
I'm not sure what logging you've setup on the phone, but are you able to see the 401 arrive on the phone?
Is it a 4G only issue? Does the Telstra phone work over WiFi?
I've ran a test here sending from a Grandstream -> Yealink Phone.
In ESL I do a 'event plain ALL', and I receive a 'MESSAGE' event
Yes, you only need a softphone running on your PC.
For example MicroSIP, which is free.
See here: https://wiki.voip.ms/article/Microsip
Under settings, set the Ring Device to your PC speakers.
Get a cheap PBX in a box with a couple of WiFi enabled phones, so it will be completely stand alone, and you won't need to run ethernet.
Something like a Grandstream UCM6300A or a Sangoma PBXact 25 will do the job.