今天看高通文档有个QMI_IMSA消息
QMI_IMSA_GET_REGISTRATION_STATUS_RSP返回字段中有P-Associated-URI。QMI_IMSA_REGISTRATION_STATUS_IND也包含这个字段。

03:42:32.818	[0x1544]	MCS QCSI Payload Packet
packetVersion = 2
V2 {
   MsgType = Indication
   Counter = 3
   ServiceId = 33
   MajorRev = 1
   MinorRev = 31
   ConHandle = 0x00000024
   MsgId = 0x00000023
   QmiLength = 97
   Service_IMSA {
      ServiceIMSAV1 {
         imsa_registration_status {
            imsa_registration_status_indTlvs[0] {
               Type = 0x01
               Length = 1
               ims_registered {
                  ims_registered = true
               }
            }
            imsa_registration_status_indTlvs[1] {
               Type = 0x11
               Length = 4
               ims_reg_status {
                  ims_reg_status = IMSA_STATUS_REGISTERED
               }
            }
            imsa_registration_status_indTlvs[2] {
               Type = 0x13
               Length = 4
               registration_network {
                  registration_network = IMSA_WWAN
               }
            }
            imsa_registration_status_indTlvs[3] {
               Type = 0x14
               Length = 76
               uri_list {
                  uri_list_len = 2
                  uri_list[0] {
                     uri_len = 55
                     uri = sip:[email protected]
                  }
                  uri_list[1] {
                     uri_len = 18
                     uri = tel:+8618211309501
                  }
               }
            }
         }
      }
   }
}

在上面的qmi消息中，对应的字段如下:

imsa_registration_status_indTlvs[3] {
               Type = 0x14
               Length = 76
               uri_list {
                  uri_list_len = 2
                  uri_list[0] {
                     uri_len = 55
                     uri = sip:[email protected]
                  }
                  uri_list[1] {
                     uri_len = 18
                     uri = tel:+8618211309501
                  }
               }

相应的空口信令如下:

从上面截图看，是广东的号码。用逗号分隔了两个URI，一个用来路由，一个用来稍后拨号。
RFC 7315定义了3GPP中的已有扩展头。这个协议中讲了这个字段。
RFC 7315 - Private Header (P-Header) Extensions to the Session Initiation Protocol (SIP) for the 3GPP
https://tools.ietf.org/html/rfc7315