First, make sure all cables to and from your DSL modem are intact and connected as described in the manufacturer's manual!
A connection can only be made if a DSL modem is synchronized with its DSL provider's counterpart. An LED will light up on your modem (usually in a steady green) to indicate such a link has been successfully established. This is why you should always check if this Sync LED (also called "Status LED" or "Line LED") is still lit!
(Please note PCI DSL modems like the Fritz!DSL Card in ADSL-WATCH use a virtual display to track synchronization status.)
In case you are experiencing repeated DSL service interruptions, you should contact your provider's technical support and have them check and fix any DSL line interference or disruption.
Fritz!DSL Card Devices
If you are using a Fritz!DSL series device by AVM, please check if the "FRITZ!web DSL PPP" virtual network card has been installed correctly. The NDISLOG.TXT file (found in the cFos directory) contains some examples and clues on how to go about that. Also, do not start the FritzWEB! DSL software! Doing so would make it impossible to connect with cFos.
You can track your modem's synchronization status using the ADSLWatch software by AVM (as shown in this screenshot).
USB DSL Modems
Some USB DSL modems may offer you a choice between WAN or LAN mode during installation. Note that cFos can only be used if such a device is installed as a LAN adaptor! Please consult the manufacturer to find out if a specific modem can be installed as a LAN device. This is, for instance, possible for the USB Router AT-AR215.
Router with Integrated DSL Modem
cFos can access a DSL modem integrated in a router only via PPPoE, which requires that the modem be visible in the network as a PPPoE device. Alternatively, some routers may allow you to configure a virtual Ethernet card.
Router as Bridge
cFos can also be used if a router can be set up as a "bridge" (like the ZyXEL Prestige ADSL; more info at http://www.your-connect.ch/news-detail.asp?newsid=89). Such a bridge will never dial into the network itself but will only relay data.