Content Types and webs.asmx

I am attempting to use th call web service action to create new content types and while the content types are created, I cannot seem to populate them with additional columns. Here is the declaritive xml that I am using for the content type:

<ContentType ID="0x0100d72cca36d77d4e36870c51dc3afc1b8b"
               Name="Voice"
               Group="Class"
               Description="Voice Class"
               Version="0">
    <FieldRefs>
      <FieldRef ID="{8536f5e3-1fef-42fb-955f-d28df9f7c921}" Name="Class" DisplayName="Class" Required="TRUE"/>
    </FieldRefs>
  </ContentType>

the referenced field is already in existence. Here is the code for the new field:

<Field ID="{8536f5e3-1fef-42fb-955f-d28df9f7c921}"
         Name="Class"
         DisplayName="Class"
         Type="Text"
         Required="FALSE"
         Group="Attributes" />

I am using the string builder to populate a viable with this xml and feed it into the call web service action. Any help getting this to work would be much appreciated.