If the you are hosting your paybill somewhere else, you have to disconnect your paybill from there.
Once that is done, inform Safaricom to remove the existing callback URL for your paybill. This can be done by sending an email to FSSSupportTeam@safaricom.co.ke and copy: email@example.com with an authorization letter specifying the URL under the shortcode/paybill to be deleted and the reason for that - the letter should have a proper letterhead, signed and stamped by top management.
Once Safaricom removes the callback, we can then host the paybill, and we will need a few things from you, learn more here.
Have more questions, reach out to firstname.lastname@example.org