One telephone line can only have one broadband supplier.
One telephone line with broadband can have more than one computer connected.
One computer can work with more than one different broadband supplier without changes (but not always).
I move my system once a week from home (Broadband with Avired - Spain) which is long distance wireless, to ADSL via Telefonica (which is via normal telephone line) at our computer club.
It just works - no changes of any kind required - nor any software for either connection. The secret is a self-configuring DHCP router/or adapter, which both connections use.