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

    <xs:include schemaLocation="error.xsd"/>

    <xs:complexType name="MakeCall">
        <xs:attribute name="to_number" type="xs:string" use="required"/>
        <!--From number does not required if it specified in login command-->
        <xs:attribute name="from_number" type="xs:string" use="optional"/>
        <xs:attribute name="subject_of_call" type="xs:string" use="optional"/>
    </xs:complexType>

    <!-- in -->
    <xs:element name="in">
        <xs:complexType>
            <xs:all>
                <xs:element name="request">
                    <xs:complexType>
                        <xs:sequence>
                            <xs:element name="make_call" type="MakeCall"/>
                        </xs:sequence>
                    </xs:complexType>
                </xs:element>
            </xs:all>
        </xs:complexType>
    </xs:element>
    <!-- out -->
    <xs:element name="out">
        <xs:complexType>
            <xs:choice>
                <xs:element name="response">
                    <xs:complexType>
                        <xs:sequence>
                            <xs:element name="ok"/>
                        </xs:sequence>
                    </xs:complexType>
                </xs:element>
                <xs:element name="error" type="errorType"/>
            </xs:choice>
        </xs:complexType>
    </xs:element>
</xs:schema>