Recent reports here suggest that you may not get any acknowledgement, and obviously you'll want to send any post by recorded delivery. If you're in a fixed term (discounted) contract you will probably be hit for early termination charges, and you'll have to hope the company manage to send an equipment return package to you.
On the plus side, you get to choose a new ISP, and can elect to have one with adequate or even good customer service. Also checkout Trustpilot for recent customer reviews before committing. Covid's been difficult for many businesses, some have dealt with it a whole lot better than others.
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