I just had this happen to me, I believe it is a result of a possible virus or malware attack, Did factory reset on phone and it is still there, which I don't believe it should be if it was never there
VoLTE means Voice over LTE so it is not WIFI Calling that's another story. It is not malware LOL.
While you're on the LTE network it will appear VoLTE enabled. You can disable it on Settings> Networks> Cellular> VoLTE/ LTE enhanced services. Try deleting and installing your APN settings to fix MMS. Your signal shouldn't be affected as it still LTE. Try switching to 3G and then again to LTE to see if it fixes.
VoLTE means Calling or Receiving Calls via 4G. Usually EE on a SIM only or a phone contract will show such icon.
3UK customers will only see the word 'HD' during a call. It shows that it is a 4G call, similar to VoLTE.