So an incoming call *shouldn�t* be an issue with no filter in place as you have it at present ....
.... but I wonder, might you try fitting a decent filter and using a shop bought RJ11 to RJ11 cable and run with it connected like that for a few days ?
I�m sure you�ll say that you don�t use it as a phone line, but it still gets rung into sporadically.... this might be injecting errors and causing the oddness you are experiencing ?
The BT master socket has a built in filter (the linesman changed the whole socket just in case, its one of the latest with no screws to fix the faceplate- just simple side clips that are pretty [censored]- too much weight on the two leads can pull the faceplate and unclip it!! It also has flick over clips to hold the incoming wires- no need for the BT chrome tool to terminate connections- linesman wasn't impressed by either "advance" in the product!
RJ11 leads are awash here at the moment, SKY sent one out special before the BT appointment, seemed pointless as the Sky operative also sent a new router which also had a new lead as well.............................!