How high will a cell phone work at? It can be a lot higher than the expected 2 to 3 thousand feet.
According to studies carried out in Canada for a single engine light aircraft it is possible - with great tenacity in the face of repeated failed call attempts - to get through up to around 7000ft or so
http://physics911.net/pdf/Achilles.pdf As height increases the probability of getting a connection linearly decreases until at around 7 to 8 thousand feet it reaches zero.