Sip

SIP = Session Initiation Protocol

Android provides an API that supports the Session Initiation Protocol (SIP).

This lets you add SIP-based internet telephony features to your applications. Android includes a full SIP protocol stack and integrated call management services that let applications easily set up outgoing and incoming voice calls, without having to manage sessions, transport-level communication, or audio record or playback directly.

On Android

see https://www.linphone.org/products