XML Schema - Elemenmt with Pattern Not Required

I have an XML schmea defined. One element type is phone, which must adhere to a pattern (see below). The value is not required, but when it is null it fails validation since it is not adhering to the pattern. How can it be defined so it only validates to the pattern when the node is not null?

<xs:simpleType name="phonextype">
            <xs:restriction base="xs:string">
                  <xs:maxLength value="10"/>
                  <xs:pattern value="([0-9]{10})?"/>
            </xs:restriction>
      </xs:simpleType>