Formula Field - Telephone and Cellphone - Contacts

Formula Field - Telephone and Cellphone - Contacts

Hi, I'm using ZOHO CRM PROFESSIONAL and inside quotes I have JUST ONE field to insert phone number.

Some contacts they have just Cellphone (Engineers) and othes just regular Landlines (Administrative people).

I've created a field called "Quotes Phone" and i need to create a formula inside this field... "IF CELL PHONE IS EMPTY USE LANDLINE IF NOT USE CELL PHONE"..


If(${Cellphone}==null,${Landline},${Cellphone}) 
String


Zoho says.. "wrong argument"... What am I doing wrong?

Thanks a lot.