While Skyping successfully to a friend who is on ADSL internet access, we are not able to use a landline phone at the same time: the landline(or Skype) drops out. What could be causing this problem? It surely should be possible to use the landline at the friend's home simultaneously with Skyping?[/B]

Maybe you have an ISP that uses your landline for Internet communication? Verify that with your ISP.

If I'm wrong, someone correct me here as this is more of a business issue that I am not familiar with.