This is NOT answered. We are experiencing the same issues with a Galaxy Lite phone.
One thing we have tried is to switch to "WiFi calling only" when at home. This is because we are in an area where T-Mobile antenna coverage is extremely weak. Although we have selected this option and restarted the phone, etc., when I call the phone at home I am usually sent to voicemail. Look, T-Mobile. If Skype can succeed with "Skype-In" then it must be as simple as making sure the phone is identified by your server when it is logged in. To blame this on a person's home network or a bad phone is ridiculous. We have tried all of the recommended "fixes" that are suggested on these blogs and on your website and none of them work. You need to fix the ID process on your end so that when someone is logged into WiFi calling YOUR NETWORK maintains the connection.