Where do you live? Callum was very popular here (north Scotland) a few years ago, both my DCs had more than one Callum in their classes, whereas Jacob was / is more unusual. (One of the Callums in DC2's class was a PITA lively child known as Bad Callum to distinguish him from the other Callums. )
Haven't heard of any new Callums recently. Perhaps Bad Callum's reputation has spread.
We're in the south east of England buy I have Scottish parents and family which is why I thought Callum would be nice but the more I say it, my Hubby doesn't like it so now we're between Ethan, Caleb and Jacob. My favourite is Caleb (but is very similar to Kaiden his brother?) and Jacob.