If the RJ11 lead shows corrosion, it is likely the filter it was inserted in has corrosion in the contacts too. I wouldn’t wait for them to cause an issue, replace them ASAP.

Yea the filter has gone anyway as now have the new faceplate but will replace the cable

If one is copper and the other aluminium you will get corrosion over time even if not wet. In a city atmosphere with SO2 present this will be faster than in non polluted air, (not sure but suspect NO will have the same effect.)

I always thought connectors and stuff were gold plated to avoid corrosion. The cable and filter were bought from adslnation back in the day so not the cheapy ones you get thrown in with the router

Ones in computers and routers maybe in expensive parts, but cables and sockets I have never seen gold plated. If both are copper there is no issue. .