"Both players are greatly talented individuals, who are highly regarded members of the squad.
"Our aim for Ulster has always been to create a competitive team focused on a core of Ulster men."
After joining Ulster ahead of the 2008/09 season, Humphreys has gained 32 caps to date and has racked up 252 points in the Magners League and Heineken Cup competitions.
From his Ulster senior debut in 2005, Caldwell has earned 69 caps for the province.
Both players have also been named in the Ireland 'A' squad to face England Saxons on Sunday in Bath.