Everytime I try to change the setting to **61*+447802091901*11*25# I get invalid MMI code. Apparently you have to disable 4g & Wi-Fi calling and then do the above sequence, once it’s done you then have to re-enable 4g&Wifi calling. Is there any other way than ringing 02 or online chat to do this. Turning them off on the phone doesn’t work, because they are done at account level. I did this by ringing 202 on the temporary o2 sim I received, now I have ported a number in it has reset the voicemail back to 15 seconds🤦It would be much easier to toggle Wi-Fi 4g calling off/on from within your account/my o2 yourself without having to wait for chat or be on hold calling my o2.