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It's a Belfast call centre selling Broadband for Vodafone.
I've had to deal with a few call centres lately - seems the operator doesn't know who they'll be talking to until you answer as (get this) the computer does the dialling and keeps calling you until someone stops it. If you answer it but there's no operator available = dead air (!) and it keeps calling back (Sky's one nearly drove me demented). The big thing in this case is it wasn't an unlisted number. I'll be calling Vodafone tomorrow directly to tell them to get my number off their system