Contacts showing as unknown number on my iPhone

Try this if your contacts are in iCloud or in your chosen email account

(You might want to back up the phone first, and if your contacts are phone only you have no remedy)

Close Messages app. then for

Contacts in iCloud:

Settings, Your name, iCloud, Turn off contacts

Contacts in another email account

Settings, Contacts, Accounts

Find the account with Contacts turned on. Turn contacts off  

When asked if you want to keep them, choose “delete from my iphone ”

Open Contacts app and it should be empty.

Power the phone off and back on.

Then

Contacts in iCloud:

Settings, Your name, iCloud, Contacts. Turn it On

Contacts in another email account

Settings, Contacts, Accounts

Find the account with the Contacts again. Turn contacts On

Open Contacts app and it should be re-populated.

Then try messages again.