HTC Thunderbolt is a CDMA/LTE phone and is not working with the GSM networks. The SIM card slot is for LTE networks (NOT GSM) therefore if you insert a GSM SIM card the phone displays "This SIM card is not recognized" error.

Additional Info:

http://support.htcimeiunlock.com/entries/20114337-is-it-possible-to-unlock-htc-thunderbolt