Hello, I'm trying to create a formula field that references 3 other formula fields, but get the "Custom formula fields are not allowed in expression." error when I try to set it up. I'm just setting up our Zoho modules and fields, so does anyone know if there is a workaround that I could use to achieve the desired formula output?

The three formulas I set up each take the area code (using Substring) from the mobile, direct, and office numbers. The new field I am trying to create first checks the direct area code. If there's a value, it will return it. If blank, it checks the office area code and returns it if there's a value. If this is blank, if checks the mobile area code and returns the value. 

How can I achieve something similar to this process in the formula field?

Thanks.