Non-graphic callouts

I have a small problem with non graphical callouts. As I have 15 such 
callouts in a listing I decided to use the non-graphical version and put 
the following in my customization layer:

  <xsl:param name="callout.graphics"          select="'0'"/>

Now the icons are gone but the numbers like (1) do not appear, neither in 
the programlisting nor in the callout listing underneath. The latter is 
indented,however, to leave some space for the numbering. 

I am rendering to PDF format using the XSL style sheets V1.62.4 and fop 
0.20.5.

Thanks for any hint,
Ekkehard