https://appleid.apple.com
Try going here and removing the phone number from the "Phone Numbers" section.
I still have friends who try to message me and their iOS assumes I'm using iMessage. The "retry as SMS" feature is the only method that works.
Also make sure to tell them "Send as SMS" is enabled on their iMessage settings. This will automatically try to re-send it as a text without manual intervention.
https://appleid.apple.com Try going here and removing the phone number from the "Phone Numbers" section. I still have friends who try to message me and their iOS assumes I'm using iMessage. The "retry as SMS" feature is the only method that works.