Not to mention the fact that according to this coverage map, the Wind 25622 cell IN NO WAY COVERS (nor comes anywhere close to covering) via della Pergola 7 (the girls' cottage) 
And as you and I well know, network coverage maps on a cell-by-cell basis are a) subject to wide margins of error (especially in a city such as Perugia, with narrow streets, hills, thick stone buildings etc) and b) by their very nature significantly overlapping (thereby making it actually impossible to draw hard lines demarcating the end of one cell's coverage area and the start of another cell's coverage area).
In practice, the only way to get any sort of clarity in this context is to actually physically go out with measuring equipment and measure signal strength and quality at the key locations, preferably more than once. For the purposes of this investigation, the key locations would include various parts of the girls' cottage (including, of course, Kercher's room); various points along the routes between the cottage, the square, the ravine, the road around the outside of the city wall, and the route up Corso Garibaldi; the park; Lana's garden; various locations within Sollecito's apartment; various locations within Guede's apartment. For each measuring location, it would be possible to see exactly which cells could be picked up, and with which signal strength and quality for each cell visible.