Lately, the router has lost connection to the exchange (variables on the dsl status page go to zero, connection takes ~1 minutes to regain connectivity) at the exact time that the phone is picked up (either due to receiving a call or picking the phone up to dial). This ocurrs when using an ethernet connection and a corded phone.
The odd thing is that the telecom helpdesk cannot see the disconnections at their end.
Is this likely to be a modem or filter issue? The filter is a proper telecom job (black case, hard wired) but it is quite old now.