I looked at the telephone cables available and found two categories, the BT CWxxxx coded products and the Internet Cat5e/Cat6 cables. The latter have a nominal electrical impedance of 100 ohms while the former do not have an electrical impedance specified in product literature, though it may well be specified in the BT specs themselves. I have not found any of the specs, only the product literature. Cat5e/Cat6 does appear to be used commonly for telephone/DSL applications so the characteristic impedance may well match, however I would like to verify this.
The BT spec. cables include the internal use flat type, CW1311, that was in place, the internal multi-pair twisted pair type such as CW1308, the external multi-twisted pair type such as CW1128, and the external CW1411 "drop wire" with a high tensile steel strainer wire.
The two pair CW1308 has the advantage over Cat5e/Cat6 of a narrower diameter and will probably suffice in our case considering that it is said to be used for DSL applications by BT and should match the characteristic impedance of the outside telephone line. I will certainly no longer use the CW1311 flat type for long range DSL.
There is more than one wire thickness option for the CW1308 and we need the 0.5mm OD wires for any RJ11 IDC plugs, though I will make direct screw in connections where possible. There are many more sellers of Cat5e/Cat6 on Ebay than those of CW1308, and the prices tend to be cheaper.
The local trade suppliers do not supply CW1308 by the meter, only in 100m lengths, though they do supply the outside cable by the meter, so I have made a purchase on Ebay.
Interestingly our outside cable was made by Wessel Cable Ltd., a company bought from Swedish ABB by UK B3 Cable Solutions in 2006, however Wessel Cable Ltd. now no longer exists. The 1988 cable carries a marking something like CPI 2P05 (the P's may be F's), but no BT spec. The local trade store outside cable is made by General Cable and has a BT spec. (CW1128 if I remember correctly). It is available locally in 5 and 10 pair form, and much greater pair numbers elsewhere.
The path of the internal DSL cable takes it to within about 18" of an 11W flourescent light bulb (this did not change recently, though a bulb drive component might have failed). If the CW1308 does not work near the bulb I will test it away from the bulb and reroute or shield if necessary.
According to a Mayo News Oct/2012 report there is a recent government plan to reach BB data rates of 30 - 100 MHz range in every home/business in Ireland by 2015 and if so internal cable quality issues can only increase.
I do not know if this plan means that every location is to have either fibre to the home/business (highest speeds), or fibre to the ("kerbside") cabinet (then copper to the home/business, lower speeds) available, nor what is Eircom's current position on the issue. A pilot operation appears to have been formally switched on about a year ago.