If ( to avoid early termination charges ) you want it to appear as though you are a home mover , taking BT at your new address avoids ETC , but anyone genuinely in that situation wouldn’t want to keep the previous occupants phone number , you are simply manipulating the system to your benefit ( nothing wrong with that )
The only way I think you could arrange what you want , would be for your parents to export their number to a VoIP provider before you move back in ( this will remove all service to the address , so be aware this won’t be seamless , there will be a break in service ) you then takeover their no longer working ‘line’ , the number allocated to you is irrelevant, it could be the number you have at your current property, or a ‘new’ number, but it won’t be your parents current number, …once your line/broadband is up and working at your parents address, you access your parents phone number ( now ported to the VoIP company ) via ‘your’ broadband connection, obviously there may be a charge from the VoIP provider for setting this up and charges for its ongoing use, and it may require some change to the way the phone is used ( it’s now a VoIP number ) but the ‘old’ number will be accessible.
