talk to retentions or wait for outbound to contact you - a bit more on the latter part of that
when a user gives 30 days notice outbound can ring to make you an offer they hope you wont refuse - usually less than you were paying - it will start a new contract so you may not be interested - its a take it or leave it deal - if you leave it you can still ring retentions and do deal with them
so sit on your hands for a week or so if you are happy to start a new contract or ring retentions and reverse your notice - not sure if that starts a new contract or not - it would if they offered you changed anything on what you had
dial 150 or 0345 454 1111 and follow the prompts to - 'thinking of leaving us' - then "technical issues" - dont worry about that they do much more - open 8am -10pm 7 days a week - should take you to a UK call centre
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Tony