A user device placing a mobile originating call on a network using Real-time Transport Protocol (RTP) ignores P-Early-Media tags and session description protocol (SDP) in Session Initiation Protocol (SIP) packets received from the network after receiving a 180 Ringing alert packet indicating that the receiving device is ringing to minimize a delay in playing a ringback tone. The user device plays a local ringtone until the user device receives audio RTP packets containing Early Media. If the user device receives audio RTP packets containing Early Media before the call is connected, the user device plays the Early Media as a custom ringtone.
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Chung, Chi-Wen; Chueh, Han-Jung; Li, Ming-Hsien; and Chiang, Chih-He, "Ringtone Regardless of P-Early-Media Tag", Technical Disclosure Commons, (November 19, 2021)