Ensure the below have been done to see if it fixes the issue.
1. Ensure you disabled iMessage. If you have already gotten rid of your iPhone you can contact Apple to have your account de-associated
2. Have your friends remove your conversation on their iPhone and start a new one. This could help resolve the issue.
3. I would also ensure your iPhone using friend has "Send as SMS" enabled in their messages section so they are not sending only iMessage messages since you won't get these.