Hi
It looks to me like that is bell wire, perhaps the developer wired it up on the cheap.
Whether it is making much difference or not you will only find out if/when it gets replaced. A lot depends on how far it runs, and if it is underground it's partly shielded from interference, but if that cable run then joins and bundles up with other similar cables, you will pick up cross talk. Now you don't pick up anymore cross talk than you would with normal BT cable, however the twists of twisted pair telephone cable help cancel out noise.
Regards
Phil