The xmlns="x-schema:phoenix.xapi" is not really a namespace. It is more like a processing instruction back from the day when XDR schemas were still used. It is instructing the parser to validate the document against an XDR schema called process.xapi.
The schema below will validate the document, but I am not sure of the runtime behaviour, this may fail in the XmlDisassembler.
<xs:schema xmlns:b="http://schemas.microsoft.com/BizTalk/2003" xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="XAPI">
<xs:complexType>
<xs:sequence>
<xs:element name="SERVICE_INFO">
<xs:complexType>
<xs:sequence>
<xs:element name="Id" type="xs:string" />
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="USER_INFO">
<xs:complexType>
<xs:sequence>
<xs:element name="EmplId" type="xs:string" />
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="INQUIRY">
<xs:complexType />
</xs:element>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>