InfoPath Dev

Hi, I am working on cascading questions in InfoPath list and trying to set fields as required when the related question appears. I am struggling with this and don't know what the best approach to set these required fields. Please see the attached my test form with the required fields that need to be set for Phone Line (check Box), Phone Number, Phone Set (check box), Type Phone Set, and Phone Set Needed. For example, if I select "Yes' for the question "Is there a phone line in the cubicle?", the Phone Number field is a required field. If I select "No", the Phone Number field is Not a required field, etc. Any suggestion is greatly appreciated. Thank you for your help as always!

Please see attached - I only added a validation rule to the Phone number field - but you'll see from that rule how to do this. Thanks for always including a sample form, it makes it so easy to help you!

 Thank you for your quick turnaround! Much appreciated. Let me try to set validation for the rest of other fields (Type Phone Set and Phone Set Needed) and see if it works. Thanks again, :)