Attenuation is a term that describes the loss of signal strength (over distance) on the 'local loop' of a telephone line. The 'local loop', also described as the subscriber line, is the physical link -- usually a pair of twisted copper wires -- that loops from the customer premises to the internet service provider's (ISPs) network. From 2000-2017, the majority of UK broadband accounts have been DSL/ADSL broadband accounts; DSL/ADSL is a technology that works on BT's stardard copper line network, which is also used to provide a POTS (Plain Old Telephony Service) / public switched telephone network (PSTN). For DSL/ADSL connections, attenuation is a measurement of signal loss (over distance) from the DSLAM (Digital Subscriber Line Access Multiplexer) -- usually located at the local telephone network exchange -- and a users premises/modem/router.
Attenuation is measured in dB decibels and the signal strength (measured in dB) degrades over distance -- you can expect to lose 10-15dB per mile. The lower the line attenuation number the better, each 3dB halves the strength of the signal: below 10dB is superb; between 10-20dB is excellent; between 20-30dB is good; between 30-40dB is average, between -4050dB is poor, and over 50dB is bad. Because line attenuation describes signal strength, it also indicates the download/upload speed a user can expect to get -- though this figure will also depends upon: whether a user has ADSL 1 or ADSL 2+; the quality of the telephone line; equipment quality at the exchange; weather conditions, electrical interference/distortion upon the line.
BT's Openreach division is currently responsible for maintaining BT's copper line network -- it is managed independently of other BT businesses, to ensure that all UK ISPs have equal access to the copper line network. The Telegraph reported in 2015, that BT has recently asked Ofcom whether it could scrap its requirement to provide POTS/PSTN services at all of its network exchanges. Apparently BT wants to move all of its domestic/business customers from traditional voice calls to internet-based voice calls. BT's plans to remove PSTN/POTS services from network exchanges/cabinets will not impact the DSL equipment installed at the exchanges.