There is a burial on Brian's index for a Jane CALLOW als BOYD 25 Dec 1793 at Braddan, which is about 3 months after the baptism of child #5 Thomas. The last 2 children of John Callow were James 1796 and Matthias 1797 so it would seem that John acquired a second wife who's still to be determined.

Matthias Callow married my 2xg-grandfather's sister Margaret Gelling in 1819 and their first daughter was named Ann (2 years after marriage so perhaps there's one ahead of her that I don't know about). There are absolutely no Ann's in the Gelling side so perhaps Matthias's mother was Ann. That could be a starting point at least, and there is a burial in Onchan 1830 for an Ann Callow but no age given. The IGI doesn't help much here. I know nothing about the other brother James Callow b 1796, nor for that matter about any of the others born earlier.

My initial guess for a potential burial of John Callow is the one at Onchan 10 Aug 1835 age 73 and I see there is a matching will. That would be worth checking.