iPhone to Android SMS/MMS text fails when using WiFi Calling

Hi all, I am having an issue sending text messages to Android users from my iPhone 8 on iOS 12.2. I get a "Not Delivered" error and can only resend when I move into cellular range again. The issue only occurs when WiFi Calling is enabled and there is no cellular signal. My wife's identical iPhone 8 exhibits the same problem. Both phones have been reset, settings checked, and carrier and network settings reset but the problem persists. WiFi Calling works fine for calls to all phones and texting between iPhones. The issue has been present for about 18mo ever since I enabled WiFi Calling and stopped using the MicroCell (At AT&T's recommendation). I have opened an Apple case and they found no issue with my phone, settings, or functionality. They suggested I open a case with AT&T. I thought I'd post here to see if others are experiencing the same problem.  Thank you for any comments.