How do I remove grayed out phone number in send/receive in message settings?
According to Apple, all you need to do is sign out of your device

 Source: https://appleid.apple.com/account/manageCheck your iCloud preferences after signing out and back in.Settings › iCloud › User Account › Contact Information

If you are unable to remove/modify

Depending on your carrier, you may need to remove your sim and reinstall iOS.