Text messages

[Maggie-Mae]

I have a Sony Ericsson K700i, which I brought second hand off ebay - it was originally an Orange phone but I was told it had been unlocked and it does accept my O2 card and works ok except for receiving text messages. They come in about 24 hours after the sender has sent them or even not at all. I have contacted O2 who have looked into it, can find no problem, they sent me a new SIM incase it was that but it still doesnt work properly. Could it be the handset - but it does send texts and makes and receives calls ok? Any ideas anyone? Thanks