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The UE shall support 3GPP R9 IMS emergency services on E-UTRAN as specified in TS 24.229 and TS 23.167.

As part of AT&T's IMS VoLTE implementation, emergency calls on EUTRAN must be supported. E911 location requirements for VoLTE are covered in 37.6, “E911 and Location,”

The VoLTE UE shall determine the XCAP root URI based on the Home Network Domain Name from the ISIM using the procedures defined in 3GPP TS 23.003 R10 section 13.9.

If an INVITE transaction fails due to the expiration of Timer B or by receiving a SIP error code of 408, 500, 503, or 504, the UE shall attempt an initial IMS registration on the same P-CSCF. If this registration attempt results in a temporary failure, the UE shall follow the procedures of <CDR-LTE-1464> Temporary Registration Failures.

CSFB shall be supported per 3GPP 23.272 when the “Voice Over PS Supported” flag is not present or not supported.

To optimize LTE VoIP performance, the “Voice over PS Supported” flag shall be supported per 3GPP 23.401 section 4.3.5.8.

VoLTE devices shall follow the procedure defined in 3GPP TS 23.221 section A.2.2 and perform a combined EPS/IMSI attach, with the setting of “prefer IMS PS Voice with CS Voice as secondary.”

To optimize LTE VoIP performance, the UE is expected to operate in a “Voice-centric” way per 3GPP 23.221.

Voice calls starting on LTE VoIP over IMS that run out of LTE coverage require supporting Inter RAT handover to UTRAN per SRVCC 3GPP 23.216.