[name]How[/name] do you pronounce the name [name]Callum[/name]? I’ve always said cal (like in [name]California[/name])…but I just heard somebody say it cay (like in cape).
I pronounce it [name]CAL[/name] (like as in [name]Calvin[/name])-um.
I live in the UK where this name is common, know four Callums (and a [name]Calum[/name]) and they all say [name]CAL[/name]-um. I have never heard it pronounced cay-lum in my life! 
Definitely [name]CAL[/name]-um (a as in apple). I know someone with a son named Caelum, which I would pronounce cay-lum.
I say [name]CAL[/name]-um. I have never even heard of it being pronounced any other way.
In Australia it’s a very common name and I went to school with several Callums. It’s definitely pronounced [name]CAL[/name]-um ([name]Cal[/name] as in [name]California[/name]). Never heard it pronounced any other way.
[name]CAL[/name]-um, as everyone has said, as in [name]Calvin[/name]… We live in the US, and our sons name gets butchered all the time! He get’s [name]KAY[/name]-lum, and CALL-um (the way people say this, it sounds like Gollum from [name]Lord[/name] of the Rings).
I’ve hearh [name]Cay[/name], but much more common for [name]Cal[/name].