Is there any way to trigger Field and Form rules based on requestor field values?

Here is my situation: We have a large number of users without a dedicated phone number of their own. I need a way to require a contact number when we put in a request from them. I have created an additional field to store this and added it to every incident and request template. I was hoping to use field and form rules to say: When Requestor's phone number is blank mandate that this field be filled in, but it doesn't look like you can trigger Field and Form rules based on requestor fields. Is there any solution for this?

Thanks!