Aluminum can be quite corrosion resistant in the right environment.  I imagine you are talking about its use as a sheathing for buried cable in soil. It should be OK as long as there is not a lot of chloride in the soil. It also depends upon the alloy, but you can be pretty sure that the cable or telephone company selected a pretty corrosion-resistant one (no Cu or Zn, low Fe) was chosen.
The aluminum is cheaper than copper and more resistant to rodents gnawing than lead.