[Q] Code unlocking problem

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http://forum.xda-developers.com – Sold my sensation xe on ebaybut forgot i hadnt unlocked it so the buyer contacted me tonight i bought a unlock code for him and he says when he enters it in the phone it unlocks restarts then says locked again has anyone ever herd of this problem or is the buyer at his work?i thought once phone accepts unlock code and unlocks that is it and there no restart etc. phone locked to 02 uk and buyer trying sim cards from 3uk and t-mobile uk. (HowTos)