This is typically caused by DSL signal issues. To check the DSL signal follow these instructions.
In your Address Bar, where you would normally type a website, erase everything. Then type in the numbers 192.168.0.1 and hit enter.
Log into your modem, click the Status icon and on your left, click on WAN status.
In the Broadband settings, check for Near and Far End CRC and Near and Far End RS-FEC Errors Interleave.
These should all have a ZERO to the right. If there are numbers over 100 here, you have a bad DSL connection and the modem is picking up errors on the line. The higher the error counts are, the more chance you will be disconnected.
Check the SNR margin. This should be 10db to 40db for a good connection. Less than 6db and your modem WILL be disconnected.
Check the Attenuation. This should be 10db to 40db. Higher than 50db and your modem WILL be disconnected.
Check the Power. The Power level should be between 10db to 40db. Higher than 50db will cause drops.
These all indicate issues with the DSL line or your phone wiring in your home.
Make sure you have no splitters or filters connected to your wall phone jack outlet to the modem.
Make sure your phone cord is between 6 and 12 feet long, no longer.
You may need to contact your ISP to have them check the lines at your house.