Your Alt Net ISP is correct in one way , a delay in provision does delay the cessation of the old service …basically it’s only once the provision is confirmed as in and working (using OTS) that the OK given is to the old provider , BT in your case , to cease the old service and raise the final bill , this is to account for delay in provision so the user isn’t left with no service ( that’s how OTS is supposed to work they basically just manage the messages between the providers ) , it’s arguably the case that the Alt Net didn’t confirm the new service in and working immediately , it was only when you called them they did the necessary action ,they then via OTS sent the OK ‘ we have control of the customer’ to BT , and that’s why the BT communication turned up soon after that .
This does mean though that the date your Alt Net actually sent the message via OTS to BT to turn off your BT service is the date you are liable for and your final BT bill will be up-to this date , ….presumably you never bothered with a phone service with your ISP using the Alt Net , if BT take a little longer to actually switch off your phone number that’s up-to BT , it’s their number , not yours , but if your final BT bill date isn’t the first day your Alt Net was working , it’s the Alt Net you need to challenge about that , that’s if it’s worth the hassle of trying to get them to credit your bill for what will likely be a relatively small sum , it’s also possible the Alt Net bill won’t start until the 25th Sept even though it was the 17th when the service was installed,
