Digital Line Detected (PBX)

I recently purchased a Studio laptop from Dell.  The are certain times when broadband access is not available to me, so I am trying to use dial up service.  I purchased a USB 56k modem, but when I try to dial out I get a "Digital Line Detected (PBX)" pop up error and the machine does not even attempt to dial.  I have seen discussions on disabling this "feature", but not for someone actually trying to use a dial up modem.  I was wondering if I disable the feature, will I be able to use the modem or is there something else I need to do to make it work.  I am also running Windows 7, and have not seen any discussion as to disabling the feature with this OS.  Any help would be greatly appreciated.