The WL-500g has support for QoS, but for Diffserv to work there will probably be some kernel options required which are not enabled (yet). Just try it and if there need to be extra kernel options, you'll have to build your own kernel (or ask another forum member to do it ).
If this seems too difficult, try getting QoS for VOIP by configuring QoS using the webinterface or with Wondershaper.