If you moved out and didn’t take the number with you , the number the line once had is made unavailable ( so anyone calling it would get number unavailable tone NU ) should the next occupant want telephone service , it’s very unlikely that they would want the same number the previous occupant had , and potentially people calling for the previous occupant, so generally there is a ‘start renumber’ order , so a new number is provided, or possibly the new occupant has moved from nearby within the same exchange area and want to bring their number with them from the old address .
It’s absolute nonsense to say you can move anywhere in the country and take your number with you , you can’t, the UK is still committed to the geographical link between phone numbers and areas , so it’s rubbish to say you can take a (for example) Newcastle number 0191 *** **** to Lincoln 01522 ***** no reputable company with telephony via their proprietary IP telephony ( like BT DV ) should offer that , however if someone ported their number to a traditional VoIP provider that geographical link is broken , you can answer a number ported to a VoIP provider anywhere in the country, so you can achieve it by this method , but it’s not something that mainstream providers should offer , the linked number scheme is still wanted by organisations like the Ofcom , and the emergency services.
As far as the OP , if you are not bothered about the number , why the interest in what happens to your old number after you move out ?
