SO, it is a wired phone, for dial tone, and not a mobile phone correct?
The dial tone voltage is killing the SYNCH signal from the DSLAM (former phone tech, I know what I am saying, trust me!)
This can be fixed with the simple filter that came with the DSL router. Place it on the PHONE, not the ROUTER.
Or, isolate the router on to its OWN line. Filter is easiest fix, and you should have one of those for each phone that is plugged into the house network!