Callum

It is never pronounced Cay-lum, certainly not in England or Scotland where it originated and is a top 100 name. It also doesn't make sense to pronounce it that way using basic rules of the English language. The "a" is short before the double "l" and the pronunciation will always be "Cal-lum." If you're looking for the pronunciation of Cay-lum, then change the spelling to just that for your child.

Could also be pronounced as Cay-lum. This is actually the only way I've ever heard it said.

Clearly you cannot read the pronunciation key for this name correctly. The first part is pronounced just like the "Cal" in Calvin. And NOT like the "aw" in "column."