I'm sorry to hear that you're not able to use WiFi calling on the Sonic network, it definitely should work.
I checked your lines connection from our end and I don't see anything that should prevent you from doing so. When you try to call out while connected on the Sonic WiFi, does the call try to ring at all or does the call fail instantly?
If it's failing instantly, then I would check to see how strong of a WiFi signal you're receiving and if it's really low, then that could be the problem. If it has full strength, but is not working, then I would suggest testing from another device to see if WiFi calling works on other devices. If it's strangely just that single device having the issue with your Sonic network, then I would try rebooting both the modem and your device, then test once more.
If you're still experiencing issues, let us know!