You need to find the marriage of William and Mary it was probably in Guernsey. They are both on the 1841 Census in Guernsey it says William was born in England there was no code for IOM so it could mean England or IOM. Lots of William Callisters born around the same time in IOM.

Unfortunately civil registration began in the 20th Century for Guernsey so you would need to search the parish records and hope William's father is mentioned on the marriage record or some other clue is included.