"Inactive SIM" K800i?

bought this phone 2nd hand and got it unlocked but screen stil showed "Inactive SIM" when i put my card in. I took this to mean the phone had an IMEI block, due to being reported lost/stolen. However, the next day it worked, with 2 different SIM cards, so i used it for a while, but after about 2-3 hours it was displaying Inactive SIM again. Presumably it would not have worked at all if it had an IMEI block on it? what could cause this? I have switched off and on, removed the battery, cleaned the SIM etc.