That is the Q!
It could your line, the filter, the router ... Are you running from test socket? Worth swapping any you can and see if it reduces errors. Don't forget to hard reboot router after any change so error counts are zeroised and we can assess their rates of change.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 20 Meg WBC