I never had an issue with Cisco ATAs connected to a cloud service behind Sonicwall devices. I don't recall setting anything up, but I am curious enough to review my setup notes to confirm.
We have not either. We have many Cisco SPA ATAs in service. All of them behind Sonicwalls. This is the only one connected to Skyetel.
Would need to see the packet as I mentioned above to try and give you any kind of answer on why.