with regard to the 2nd broadband affecting speed on both services, my gut feeling is that the normal phone cable(s) are not twisted pairs, or shielded, so until the wire turns into these or goes to optical there is a good chance of crosstalk interference.
Just a gut feeling based on 20 years of various electronics and cabling etc but I could be wrong about current phone cable constructions or other mitigating technologies.
Normal telephone cabling as installed by BT in the street and to the home is twisted pair and has been for many years.
Some very ancient decades old drop wires may be untwisted pair, but usually just two wires and support for no second line. So a new drop wire would be installed, complete with twists