I've tried 
<https://api.linkedin.com/v1/people/~/connections:(id,first-name,last-name,phone-numbers)>
but it's only get (id,first-name,last-name)

even this: 
<http://api.linkedin.com/v1/people/id=UserId:(phone-numbers)>

Does LinkedIn expose this field ? I've google it, but not found somewhere ..