Have verizon at home and iphone 5 so didn't need to call in advance to unlock my phone. Couldn't purchase the sim in advance. Lebara wouldn't mail mine to USA. So I waited and purchased in France. Happened to buy mine in Provence for €10 at a tabac shop but forgot to ask for a nano chip. Not all the shops carry a nano chip for iphone 5. When I got to paris, kept looking but again no nano chip or the shop didn't carry lebara.
Since I had nothing to lose, I decided to grab a pair of scissors and cut the micro sized chip to size. Was able to activate and use. Luckily I still had the package it came in (very important - your new number is on the package and is required for activation).
Chose lebara because I could roam with it in the UK. Worked well.
Hope this helps someone.