Telephones are connected back to their exchange via a pair of wires in a cable. Such pairs are always twisted so that each wire remains close to its partner.
Most small cables have a core of a small number of pairs twisted together, followed by layers of pairs which then twist in opposite directions and have different lengths of twist or "lay".
Internal cables are generally finished with a white or grey PVC sheath. External cables usually have black polythene or PVC sheaths to resist sun and ultra violet damage.
Telephone circuits can be made up of many different cable lengths . However all the cables in the circuit need to have the same characteristic impedance. If a change of impedance were to occur, then some reflection of the signal would occur at that point and this would cause a significant loss in the line. Cables used for telephony generally have a characteristic impedance of about 1200 ohms. Cables designed for data work often have lower characteristic impedances, and although often employed for telephone connections are not really suitable over longer lengths.
Cables start at the exchange "Main Distribution Frame" or "MDF". They are terminated on fuse mountings on the line side of the MDF. It is useful to remember that external cable pairs generally "count down", both on the MDF and at DPs. (Note that internal exchange cables generally "count up" when they terminate on frames).
Note that any point in the cable routing the "E" side looks back towards the exchange and the "D" side looks on towards the telephone.