If you are looking for a new mobile phone I cannot emphasize how important it is for you to forget about even walking into an O2 store.
They provided inaccurate information about tariffs meaning calls cost 5 times as much as quoted resulting in incredibly high charges after just day one..
When asked for a refund was told that it isn't possible because "they didn't do anything on purpose" and that they "can't give refunds anyway".
The customer service staff available online, in the call centre and in the store are all absolutely incompetent, have zero customer empathy and are more than happy to speak to you in a highly sarcastic and derogatory way.
Once they've got your business they could not care less.
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