Honestly, I have no idea! I'm not familiar with the FONA sorry, but I'll see if I can poke someone who is!
Apparently it's supposed to be a "2G SIM", but I thought all SIMs included GSM support. Adafruit's docs only cover the USA really:
https://learn.adafruit.com/adafruit-fona-808-cellular-plus-gps-breakout/obtaining-a-sim