It's most likely the splitter. Plus you may have too many filters in the ADSL line. My Router insists the order is: Router directly connect to ADSL line without any filter (or splitter!), the telephone is then connected to the phone out socket on the Router with a filter between the two but after the Router. Filters added to any extensions - but that assumes you are one the Primary BT socket in the first place. So it's Router directly to the ADSL line then filter afterwards. And take the Splitter out and fit it after the Router - if you must have it.