That is an interesting thing to have, but i don't think Webform support such a functionality. However they are some other way to achieve the same result.
Using Webform Conditional module. you can have two sets of hidden questions, and only one set of question could be visible base on user selected choice.
If user click on phone the first set of question will be display to user where the phone is a mandatory and email optional.
If user click on email the second set of question will be display to user where you can have email as mandatory and phone as optional.
You can ready more about alternative ways here .