Make sure your phone line is working perfectly, i.e noise/crackle free, before you log a fault to your ISP.
Call BT on (0800 800) 151 for telephone line faults once you have checked your own telephone equipment and wiring as much as you can.
The ISPs can only guarantee a reliable ADSL connection at the NTE5/ master phone socket for your home. Any extension socket or cable will increase the risk of the connection being affected by electrical noise (wiring and appliances in your home and surroundings) and this will in turn increase or even be the sole reason your ADSL keeps disconnecting.
If you remove the 2 screws on the faceplate of your master socket, you should see what is known as a test socket. Plug your filter and modem/router directly into this and then monitor your ADSL connection. Using the test socket eliminates all other telephone extensions in your house and will help you and the ISP/BT in determining where the fault lies.
If you still experience disconnections at the test socket having tried a different microfilter and modem/router then call your ISP, explain all the things you have tried, and they should escalate the fault to BT Wholesale.