Either one is required on form..

Sometimes we need either phone or email is required to be filled. 

To do this mostly we will write JavaScript, but we can do this by using CRM business rules also.. 

You need do this by adding three different business rules on Phone and Email attribute. 

Here are these business rules

1. When user opens form, on form either you can show message that Phone or Email required and set Business requirement level. 

2.  If email entered, then remove Business requirement from Phone.

3. When Phone entered email is not required. 

Done, save your business rule.. publish..