I want a Numeric formula field to be blank

I am using one form to calculate two values (emails and phones). There are two fields to calculate these number, 1- Sum of emails, 2- Sum of phones. If i'm calculating emails I need the Sum of phones to be blank not 0. Is there any way to achieve that. I tried an If formula, so if the type is Phone the value of the field to be "", but it's giving errors. 

my formula:
If(
="Phone calls","",
=-100,0,
=-100,0,
100-)