Final answer:
Interference between DSL signal and voice signal sharing the same copper telephone line is likely causing poor telephone communications, which can be improved with filters and proper wiring maintenance.
Step-by-step explanation:
The most likely cause of poor telephone communications when using a copper telephone line that also has a DSL service could be due to interference between the DSL signal and the voice signal, which share the same copper line. This situation can be significantly improved through the use of filters or splitters that separate the signals, ensuring that the data transmission of DSL does not interfere with the voice calls. Additionally, deteriorated wiring or too many devices connected to the line without proper filtering can exacerbate the issue.