I successfully did this using radio box field following this code

https://formidableforms.com/knowledgebase/frm_validate_field_entry/#kb-conditionally-require-a-field

I changed my mind about using a radio box because my field was "How would you like notifications" with 3 radio choices, email, text, both (email and text).  My form is getting too messy so I thought I would just change that field to checkbox with two choices, email and text  The end user could also choose one or both.  

The issue I'm having is I wanted form validation so if the user chooses email notification, he must enter an email address in the email field.  If he chooses text notification, the user must enter a phone number, and of course, if the user checks both, he must enter an email and phone.  

The code from that KB document doesn't work for the checkbox field for some reason, and I can't figure it out.  Does anyone have a suggestion?