Presumably since the others complied with instructions to turn them off. Jets fly 5-6 miles above the earth, and cellphone signals propagate along line-of-sight paths, so theoretically could be received anywhere within the distance you could see from a window seat. Doesn’t narrow it down much, but it is another data point.
My understanding is that cell phones won’t work above 7000ft and even if they do connect a conversation is not likely to happen.
The co-pilot’s cellphone may have been turned on earlier in desperation while at 45000ft and just stayed on after the co-pilot was incapacitated and then connected when the plane dropped below 7000 and came within range of the tower.