they have stopped doing boosters - the new thing is pods - same but different - if you are on the 1gig package or the top range deal then they are free - for others they are £5 a month - you can have 3 so dont know if thats £5 total or £5 each
either way if you have to pay for them you are better looking at something that you buy outright and own rather than a mothly cost for the rest of your contract
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Tony