1. Check how many telephones you have in the house as you may be exceeding the REN. Unplug all and then try to connect, if no problems you are exeeding the REN.
2. Uninstall and reinstall your dial up networking within windows control panel Add/Remove programmes, then see if you can connect.
3. Check your dial up settings with BT.
4. Could be a line fault, contact BT and report it.
5. Update your modem drivers