In the event the UE receives a 380 (Alternative Service) response to an INVITE request 
the response including a 3GPP IM CN subsystem XML body as described in subclause 7.6 that 
includes an <ims-3gpp> element, including a version attribute, with an <alternative-service> child element with 
the <type> child element set to "emergency" (see table 7.7AA), 
the UE shall automatically send an ACK request to the P-CSCF as per normal SIP procedures and terminate the session.
 In addition, if the 380 (Alternative Service) response includes a P-Asserted-Identity header field with 
a value equal to the value of the last entry on the Path header field value received during registration:
-    the UE may also provide an indication to the user based on the text string contained in the <reason> child element 
     of the <alternative-service> child element of the <ims-3gpp> element; and
-    one of subclause 5.1.6.8.3 or subclause 5.1.6.8.4 applies.
NOTE 2:    Emergency numbers which the UE does not detect, will be treated as a normal call.
NOTE 3:    The last entry on the Path header field value received during registration is the value of the SIP URI of the P-CSCF.

 

当UE正常发送INVITE收到380响应后满足上面标黄两个条件时就会重新尝试IMS ECC。