Probably the simplest thing to do would be port the business number to a traditional VoIP provider, this should close down your business account, you don’t interfere with your home broadband ( for the time being leave it as it is ) , some VoIP providers supply a preconfigured ATA ( analogue telephone adapter ) that you connect to the BT router to access the business number over your residential broadband ( so you effectively have two phone numbers , the residential one via the socket on the BT router , and the business number via the ATA ) or they give you the configuration information to configure an ATA that you purchase yourself ( you can buy phones with a built in ATA ).
What you want , as far as renumbering your residential phone number to the business phone number and allowing the residential number to ‘disappear’ , probably is still possible, but not worth the frustration trying to get it done .
If at some point in the future you want to get rid of the residential phone number , when your broadband contract is up for renewal, you simply renew as a broadband only customer and the residential number is removed.
