iMessages lost from caller after call cut off with horrific shrieking sound

When my mother and brother spoke on their iPhones (both running iOS 11) there was suddenly a very loud shrieking sound, which my mother described as dragging the edge of a snow shovel on asphalt.

Then the call cut off. It was a normal mobile call, not over Facetime or anything else.

When my mother looked, she had lost all iMessages from my brother, but not from anyone else.

Now my mother is wondering how to get the messages back. They are on her computer. Every night she backs up to iCloud via wifi when charging the phone so they ought to be in iCloud too.

Is there any way to restore the messages from my brother to the phone?

Thanks in advance
Philip