Unlocked Samsung Galaxy S8 Plus no longer allows mobile data when on call?? Why, Please help

[Android Central Question]

I have Straighttalk service with an unlocked Samsung Galaxy S8+ that I purchased at Best Buy. I have had it over a year and always been able to use mobile data when on a call. Approximately a month ago (roughtly the end of Feb) it quit letting me do it. Straighttalk is absolutely no help whatsoever, if I happen to get a Rep on phone that I can understand then there is still a language barrier because they apparently can't understand what I am saying as they continue to say if my data is working and I can place and receive calls then my service is working?? Does anybody have any ideas for me? There is no option of voLTE or Advanced Calling in my setting. Nothing on my phone has changed except for what I have explained. No 3rd party apps, no change of sim cards, absolutely nothing ? I am ready to scream because I need that feature and spent entirely way too much on this phone to have such restrictions. Please help

 

[belodion]

Moved to S8 for comments from users.