How do I get a chained restful API call to return xml?
@samuelsov got me going in the right direction, along with trial and error from the documentation:

This returns all related phone records for the given contact_id:

entity=Contact&amp;action=get&amp;contact_id=1234&amp;api.Phone.get[values]


And you can filter the chained API call too, for example all related phone records with a phone_type_id of 2:

entity=Contact&amp;action=get&amp;contact_id=1234&amp;api.Phone.get[phone_type_id]=2