<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

    <xs:complexType name="Request">
        <xs:attribute name="client_id" type="xs:string" use="required"/>
        <xs:attribute name="api_key" type="xs:string" use="required"/>
        <xs:attribute name="phone_number" type="xs:string" use="required"/>
    </xs:complexType>

    <xs:complexType name="Response">
        <xs:all>
            <xs:element name="ok"/>
        </xs:all>
    </xs:complexType>
    <!-- in -->
    <xs:element name="in">
        <xs:complexType>
            <xs:all>
                <xs:element name="request" type="Request"/>
            </xs:all>
        </xs:complexType>
    </xs:element>
    <!-- out -->
    <xs:element name="out">
        <xs:complexType>
            <xs:all>
                <xs:element name="response" type="Response"/>
            </xs:all>
        </xs:complexType>
    </xs:element>
</xs:schema>