That sounds... Improbable. When you were passing over Iceland you must have been far too far above the antennas to receive any sort of signal, unless it landed there on the way across the ocean?
A phone at cruising altitude can see and try to connect to a huge number of cell towers at once. Like, orders of magnitude more towers than it would be able to see even from the top of a mountain. Plus, you're constantly disconnecting from some towers and connecting to new ones. This puts a strain on the cell network systems.
One person won't have a negative impact but then you multiply the problem by the number of people who fly daily.