UK here.I have seen both spellings used equally and they were both very popular. I prefer Calum because I really like Cal on a boy.I do pronounce Calum with two distinct syllables. CAL-um (does not rhyme with Caleb which is CALE-ub). The Cal is short like Calvin.
I like Cal too! Ahh that is good. I once met a guy called "COLM" (that is roughly what it sounded like) and I thought it might be Calum said really sloppily, but maybe he was Colm after all :)