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I was a Vodafone customer for many years as was most of my family but after being offered an iPad for a great monthly price as an additional line and being sent out a regular tablet I contacted Vodafone, after calling to complain and cancel the tablet and waiting for the call to be listened to I eventually heard back saying they apologised and this should not have happened etc, which I accepted but I wanted to tablet line cancelled. Shortly after this my phone was also disconnected and I received a letter demanding over £300 to settle my phone contract which should not have been cancelled, I have sent numerous letters that were signed for by Vodafone staff and received no reply and they have issued a default account on my credit file for this and still had no reply to around 10 letters.
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You may find some helpful info in >Default-on-your-Credit-File-and-How-to-add-a-Notice-of-Correction.