If you moved , you effectively lose any ‘ownership’ of the line at your old property ( why should the new occupant have to wait around for someone who has left the property to eventually get around to doing something about it ) so probably , they new apply for service ( with anyone using Openreach network ) your ‘old’ provider sends a letter to that address saying is it ‘OK’ or a mistake for the line to taken over, you obviously are not there to answer that letter, and an unanswered letter is deemed consent that the takeover is OK, so the new customer at your old address , effectively ceases your service and if you haven’t already made alternative arrangements with BT at your new address, eventually you will get a final bill , the last date being the day before the new customer started service with whatever company they used.
Your number from that address will now be back in BT quarantine, and when sufficient time has expired, it’s available to be re allocated to another BT customer, any opportunity to port the number has now gone, plus number port isn’t a home mover service, it’s to remove a barrier for changing providers at the current address, it’s not intended to be done retrospectively, addresses need to match up when a port order is arranged, obviously your new and old addresses are different.
If you are willing ( if not happy ) to get a BT line installed at your new address and both the old and new address are served from the same local exchange, then you ( eventually ) will be able to get the number back ( getting whatever new number is on your new BT service , renumbered to the number you want, ) it’s not immediate, although there should be no extra delay due to ‘porting’ complications .
If you confirm you have had a BT line installed at the new address, and both addresses are connected to the same exchange, then a BT mod may offer to arrange the renumber, if those prerequisites are not met then there isn’t anything to be done, presumably you must have realised that if you leave a property but leave an active account there , then not only will it inconvenience the new occupant, you have no control over what happens next, you should have made arrangements before you moved , not after.
Whoever the ‘alt net’ you used for broadband are ‘responsible’ in no small part for this situation, because the advice they gave you had at least two errors , number port is not the correct process if you are moving home , and they shouldn’t say it’s OK to not advise your current provider about you leaving the original property.
