At Terminal 3 last time I was there, it was best to sit right by the Vodafone stand. 3G was very good there, unsurprisingly, but is generally good across the airport. Fixed line was surprisingly good, including some stand-up terminals that you pay for by the pound, though these don't allow you to plug anything in - more for just a quick email over webmail. Horrible seats and over-crowded, as usual, but no one seemed to care about me recharging my laptop while surfing.