Hey sleaudio! Utilizing Wi-Fi calling is a life-saver for many people, so this is definitely something we want to help you resolve. If the error you're seeing is REG05, then that fix is a little different - according to our Wi-Fi Calling troubleshooting error codes, your SIM may just need an update that we can send it OTA. If you're seeing "ER05 Invalid SIM", then you may need a new SIM card as this error displays when a SIM is not enabled for T-Mobile Wi-Fi calling (or not a T-Mobile SIM!). However, on some occasions, this can also be caused by provisioning and solved simply by Contacting Us and letting us check the settings on the back end and possibly send an OTA to the device. We'd definitely like to give that a shot first, so please reach out so that we can take the necessary steps to get this working for you! Just a heads up - you'll need to be in an area with T-Mobile coverage to receive these updates over the air!