A VoIP network (Skype, Yahoo Messenger with Voice, SIP, etc) should be compared to any PSTN network because of its base function to allow phone calls from user to user. Though, the services available on PC-to-PC VoIP networks are scarce compared to services available on regular phone networks.
For instance I don’t know any Skype contact or Gizmo Project number that I can call to listen to a joke. Although limited the services available for VoIP networks are still there:
echo123 - Test your connection reliability.
For more voice services for Skype try browse their directory at: http://share.skype.com/directory/voice_services_?c36/50/0/title/ASC/
Gizmo Project / SIPPhone
** - Hear your SIPphone number repeated back to you.
*0 - Test your router's SIP compatibility.
411 - The voice-activated Tellme information service.
1-747-474-3246 - Echo Test - Repeats back whatever you say.
1-747-474-5000 - SIPphone welcome recording
1-747-555-2663 - SIPphone Conference.
1-222-000-0000 - SIPphone party line.
sip:email@example.com – Listen to media channels of audio entertainment