Cell phones generally send out data ever so often (ever few seconds, I believe) that says "I am here." This is to let the cell towers know where the cell is. What happens in a city is that the phone will ping, and the surrounding cell towers will listen and compare notes. The phone is then instructed to communicate with the strongest (clostest) tower.

In this case, it would appear that one single ping was able to, by chance, get through. But it was not reliable enough to get a conversation going.