Diane,
I was looking on the BTL site for something else but came across this info.
Group Hunt
Group Hunt provides your business with a tool in ensuring that you provide the necessary service to your clients. You are provided with several phone lines, however only one number is advertised. When your clients call the number, if it is busy, the call will roll over to the other phone lines provided.
So it looks like we do have that type of service after all.