Django REST Framework - POSTing foreign key field containing natural key?

I've recently started using the Django REST Framework (and Django, and Python - I'm an RTOS/embedded systems person!) in order to implement a RESTful Web API. Haven't had any problems yet which couldn't be resolved with Google, but this one has had me stumped for a few hours now.

I have an embedded system which listens for events which are associated with a range of devices - analogous to a Phone making Calls, which is what I'll discuss here for brevity. A Phone has a number and a whole lot of Calls (that it has made) associated with it. A Call has an associated Phone (the Phone which made the Call) and a time of creation. When a Call occurs, it should be POSTed to the API. I have an embedded system which listens for Calls and their originating phone number, and submits them to the API. Since the embedded system knows the phone number, I would like it to submit: {"srcPhone":12345678} rather than {"srcPhone":"http://host/phones/5"}. This avoids the need for my embedded system to know the primary key of every Phone (or to GET Phones by number every time it wants to submit a Call).

Google and the Django docs suggested I could achieve this with natural keys. My attempt follows:

models.py

from django.db import models
from datetime import datetime
from pytz import timezone
import pytz
from django.contrib.auth.models import User

# Create your models here.
def zuluTimeNow():
    return datetime.now(pytz.utc)


class PhoneManager(models.Manager):
    def get_by_natural_key(self, number):
        return self.get(number=number)


class Phone(models.Model):
   objects     = PhoneManager()
   number      = models.IntegerField(unique=True)

   #def natural_key(self):
   #    return self.number

   class Meta:
      ordering = ('number',)


class Call(models.Model):
    created    = models.DateTimeField(default=zuluTimeNow, blank=True)
    srcPhone   = models.ForeignKey('Phone', related_name='ca