I'm not completely sure I understand your problem. I think you want to use LEFT JOIN
to see if the record exists in the phonebook table.
And perhaps use IFNULL
to show the name or the sendernumb:
SELECT i.sendernumb,
IFNULL(p.name,i.sendernumb) name,
i.text text
FROM tbl_inbox i
LEFT JOIN tbl_phonebook p ON i.SenderNumb = p.number