1. It's important that you plug a microfilter into each telephone socket that is in use on a line that has an ADSL service. You don't need to plug a microfilter into sockets that have neither a telephone nor your ADSL router/modem plugged in to them. Don't "chain" microfilters. In other words make sure that there is only one microfilter between your router/modem and the master telephone socket. For example, if your router is plugged into a telephone extension which is in turn plugged into a master telephone socket, do not plug a microfilter into both the master socket and the telephone extension as this would mean there would be two microfilters between the ADSL modem and the telephone line. Be aware that if you have a Sky Digibox or any other set top box, a burglar alarm or indeed ANYTHING that plugs into your telephone line, a microfilter MUST be plugged in at some point between such devices and your master telephone socket.
2. Having too many devices plugged into a telephone line can cause problems. Similarly some devices may be incompatible with the ADSL service. Try removing everything except the router/modem from your telephone line. Try plugging your router into the master socket rather than an extension.
3. Try making a call on your telephone while the router light is flashing. Calling any number will do. If the light mysteriously stops flashing then you probably have a problem with your microfilters or the wiring for your telephone lines may even be faulty. The light may stop flashing if you have "chained" microfilters.