Signed by former Blues boss Trevor Francis from Sheffield Wednesday in August 2000, Sonner left St. Andrews last summer having not been offered a new deal by Blues boss Steve Bruce and moved across the Midlands to Walsall.
However, after less than a full season at the Bescot, the one-time Northern Ireland international had his contract paid up by the Saddlers.
A free agent, 31-year-old Sonner is now hoping to kick-start his career at the City Ground.
