Wifi Calling on my Samsung S7 Edge was killed by the 1st July Update, however with todays update it has now been reinstated along with VoLTE on BTU!
Last software upgrade brought Wifi calling back before replacement SIM arrived, so don't know if new simm would have helped
Can I ask how you managed to get this working on an S7. I am trying and contacted BT but no luck!
Flashing a handset is something you should only do if you are confident in doing so, if you do go down this route ensure that you back up any data on your handset. Also BT do not gaurantee that the service will work even if you flash the device.