That is true for most of the carriers. ATT is the most ridiculous carrier from that perspective because blocks all of the "advanced" calling features on non-ATT-branded unlocked phones such as WiFi calling, video calling, VoLTE ... Even you loose visual voicemail. On Verizon you would only have VoLTE and visual voicemail working, the rest will not work. Not sure about Sprint. T-Mobile supports VoLTE and visual voicemail, not so sure about video calling and WiFi calling.