@guru I had the “X-Cell Dynamic IMEI v3.1” They are still available at the following link (only their website that sells their Android phone was taken down). https://x-cellular.com/phones.html
@dcz Not nitpicking at all. I appreciate the insight. I am dubious about everything security-related. 
I will say that their customer service was very responsive and helpful long after I purchased the phone and I had the sense that they were knowledgeable and sincere in their efforts.
I have no idea how SMS operated under the hood. I was just guessing.
I do think their philosophy about stealth vs encryption warrants consideration. They discuss it here: https://x-cellular.com/about.html