[name_m]Liam[/name_m] and [name_m]Callum[/name_m] just read as separate names to me. I think with [name_m]Liam[/name_m] being so common as a full name, it adds to the confusion if used as a nickname for an unrelated name.
If [name_m]Cal[/name_m] isn’t for you, [name_m]Cam[/name_m] is a good alternative.