First of all, your original provider shares at least some of the blame for this. The transfer process has a minimum 2 week “cooling off period”. This is to allow your current provider to retain your business if necessary. When the order was placed by Vodafone your provider would have been notified almost immediately, giving the opportunity for them to contact you to find out why you were leaving. Although it doesn’t explain why your number changed, as that typically is the key identifier in placing any migration orders.
It sounds to me like Vodafone has carried out a working line takeover on your line, hence why the number changed. Basically they place an order Its not uncommon in multi occupancy premises, but it should have been picked up at the time, rather than after the event.
As a provider its an utter PITA to get things back to how they were before and two weeks isn’t bad going.
As the wise cougar has mentioned, why something so critical to your business (costs you thousands of pounds for downtime) doesn’t have a backup for exactly this type of scenario. For the sake of £50 a month you’ve lost thousands, which isn’t good planning really.