I don't think it is your SIM if it works in your old phone. I wonder if the phone is truly unlocked, or if it is not fully compatible. You might contact Sony Ericsson. That model is an international unit.
I have Cingular service and recently bought a unlocked sony ericsson k700i phone from ebay in January. I put my old sim in the sony phone and it worked great for about 2 months. Then this "insert SIM" message comes up and I can't access any phone functions. I put my sim back in my old phone and it works fine. Then if I put my sim back in the sony phone it works fine for about an hour, then the "insert SIM" message again. Is there a way to get rid of this annoying message? I called Cingular and they told me to clean the contacts with an eraser(which I did). The ebay store I bought it from isn't responding. I can't figure out if its the SIM, the phone or both? Any help is appreciated.

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