the Dect phones vary. Some say max clear line of site is 50m, other models are up to 300m. How that would translate to real world putting in a location well away from the cottage depends on the location, but you'd expect a 300m model to stand half a chance.

100 yds of 'bellwire' on a socket of unknown provenance might also cause problems (you'll need socket extension wire, not bellwire! and that is one hell of an extension for a normal phone setup). If in rented cottage, I'd guess the chances of being able to dig it in are low, and probably draped across the ground for the period they are there. Long runs like that can cause problems though.