xsd validation for tag value not null

My Xml is :

    <LineService>
        <LineSrId>SR_CP_JD_144084-1</LineSrId>
        <MSNumber/>
        <AccountNo>1320116817829261</AccountNo>
    </LineService>

And my xsd is :

<element name="LineService" maxOccurs="unbounded" minOccurs="1">
    <complexType>
        <sequence>
            <element type="string" name="LineSrId" maxOccurs="1" minOccurs="0" nillable="true"/>
            <element name="MSISDN" maxOccurs="1" minOccurs="0" nillable="true">
                <simpleType>
                    <restriction base="string">
                        <pattern value="\d{10,18}|"></pattern>
                    </restriction>
                </simpleType>
            </element>
            <element type="string" name="AccountNo" maxOccurs="1" minOccurs="0" nillable="true"/>
        </sequence>
    </complexType>
</element>

I want to check MS Number or Account No can be null but both should not be null for a request. At least one of them should be not null and having values.