At least here in the UK, phone numbers are generally network specific - including mobile numbers. Companies don't just reassign ownership, the original owner of

At least here in the UK, phone numbers are generally network specific - including mobile numbers. Companies don't just reassign ownership, the original owner of the number has to actively forward calls. This causes some issues when a provider goes out of business and Ofcom or whoever have to call around and ask nicely for someone to take on forwarding for their number ranges. (There's talk about fixing this with a big central database, but I don't think it's come to fruition yet.)