Here is everything you need broken down. You can simplify if you like but you get the idea:
IIf(Not Isnull([CallingContactRel]),[CallingContactRel],"") &
IIf(Not Isnull([CallingContactRel]) and nz([BookingNoteContactExcl],0), ": " ,"") &
IIf(nz([BookingNoteContactExcl],0),[CallingContact],"")