imessage on iphone uses sms to one specific iphone

I have a situation where the iMessage traffic between iPhone and another iPhone in the same house, on the same Wi-Fi only uses SMS. I can text this phone from my iPad and it uses iMessage/IP but for this specific phone it’s all SMS. I’ve checked the iMessage settings in both phones and iMessage is enabled.

I ran a test and sent a test from my iPad, immediately followed by my iPhone and the iPad used IMessage protocol and the phone used SMS. All involved units are on the same

WiFi here in the house. Cellular data is on as well for the phones.

This suddenly started happening around mid day yesterday. The phones are on the latest release of iOS, which is 15.4 but that happened several days ago.

Has anyone experienced this? I am thinking of deleting the message history and the contact entries on both phones and re-creating them to see if that solves the problem but I hate to lose the message history.