I have a Naked ADSL connection (no dial tone, connects to ADSL internet only) with an ADSL router plugged directly into the wall. All calls are made using the VOIP ports on the ADSL router. Should I need to place an in-line filter between the VOIP port and the phone?
No, you do not need to use DSL line filters on phones attached to VoIP equipment. These should be used only to phones connected to your landline connection (which you don't have with naked DSL).