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Found yesterday morning on Great Victoria Street. It's locked with a password, but I have the IMEI number and of course the sim card. It's been on since I found it but I've not had any calls or messages yet. I'd turn it into police but I suspect they have more important things to do and I'd really like to get it back to its owner.
You could take it into a Vodafone shop if there's one handy. If it's a contract phone, or the owner has registered it, it may be possible to contact them. Otherwise, I'd go to the Police with it. If it was my phone, that's the first place I'd look to see if it's been handed in.