It works with the Castel Cloud SIP server through Wi-Fi and 3G/4G/5G. Main features: - Audio and video calls with advanced functionalities (Loudspeaker, transfer calls, hold...) - Address book with favorites - Call history - Remote commands during calls and DTMF - Support landscape and portrait mode for all smartphone and tablet designs (camera notch...) Note: Please, check your cellular operator's terms of agreement to make sure they allow SIP calls on their network before using Castel SIP App.