The phrase "call upon the name of the LORD" turns up occasionally in Genesis:
Genesis 12:8 (ESV)
8 From there he moved to the hill country on the east of Bethel and pitched his tent, with Bethel on the west and Ai on the east. And there he built an altar to the Lord and called upon the name of the LORD.
Genesis 13:4 (ESV)
4 to the place where he had made an altar at the first. And there Abram called upon the name of the LORD.
Genesis 26:25 (ESV)
25 So he built an altar there and called upon the name of the LORD and pitched his tent there. And there Isaac's servants dug a well.
I've read a couple thoughts on it that suggest it means humans now had to strive to know God. The first time it's used seems to be unique (see a related question on Genesis 4:26). Do the subsequent uses of the phrase call back to the first or was it just a standard formulation that meant "worshiping God"?
Either way, does anybody know what it means to invoke on the LORD by name?