Reasons for slow sync with the CO:
1. Telephone surge suppressor in the circuit between the modem and the phone jack, such as those found on a UPS or power strip.
2. No micro filter installed between phones and the telco line. (Applies to splitterless installations.)
3. Poor line quality.
4. Problem with modem in CO.
To troubleshoot slow sync, follow these steps:
1. Disconnect all phones and other equipment from your phone jacks.
2. Make sure that the phone line connection to the modem is direct. Do not wire through any surge suppressors.
3. Power up the modem. It should sync with the CO within 5 or 10 minutes.
4. If it still won't sync, call your ISP to report the problem. Your phone line has it's own modem at the CO end. There could be a problem with it. Or there could be a problem with your line.
Once your sync time is OK, plug in each of the phones or other equipment one at a time and check modem sync by power cycling your modem. With splitterless setups, make sure there is a micro filter between each phone and the phone jack.
Also read About DSL for lots more information