Its a question for your new provider, although it’s true that ported number are returned to the original number range holder , if the provider who had ported it to themselves, relinquishes it ( says ‘we don’t need this number anymore’ ) , unless you or your mother decide the number is so important that you sign up with BT again , then BT have no involvement, and it’s the case , unfortunately, even if you returned to using BT there is no guarantee the number can be reallocated,
if the number was to remain ported the new provider is responsible for than , not the original number range holder , it sounds like they either don’t port numbers ( they don’t have to ) or they said they would but didn’t, the result is the same though .
Ofcom now requires numbers to be available for 30 days after cessation ( which is what the number effectively is ) , so in the past this window didn’t exist , but it still needs your new company to arrange it ,so there is a chance that your new ISP could retrospectively ask BT to port ( they ask , not you by asking BT ) , if they say No ( or lie and say it’s not possible as it’s too late ) that’s between you and them ….sorry if that seems harsh , but if BT are not the supplier it’s really of no consequence to them your issues with your ISP
FWIW , if you never asked to retain the number, and assumed the number would be retained, that’s your error , if you requested they did retain the number , and they didn’t arrange it , that’s a criticism of the company you elected to use
