overriding form field queryset

class ExtensionForm(forms.ModelForm):
    xmpp_buddy_groups_choices = xmpp_buddy_groups.objects.values_list('group_name',flat=True)
    class Meta:
      model = SipExtension
      fields = '__all__'
      widgets = {
            "xmpp_buddy_groups_names": forms.CheckboxSelectMultiple
        }
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs) \
            DidNumber.objects.filter(did_owner_extension=self)   # THIS DOESNT WORK






# extensions/models.py
class SipExtension(models.Model):
    sip_extension = models.PositiveIntegerField(unique=True)
    sip_secret = models.CharField(max_length=32)
    commlink_push = models.BooleanField(default=False)
    real_name = models.CharField(max_length=32)
    default_did = models.ForeignKey('dids.DidNumber',null=True,blank=True)
    xmpp = models.BooleanField(default=False)
    xmpp_username = models.CharField(max_length=50,default='username')
    xmpp_password = models.CharField(max_length=32,default='password')
    xmpp_buddy_groups_names = models.ManyToManyField('xmpp.xmpp_buddy_groups',blank=True)





# dids/models.py
class DidNumber(models.Model):
    did_number = models.CharField(max_length=50, unique=True)
    did_owner_extension = models.ForeignKey(SipExtension,default=None,null=True)   # I want to filter the ExtensionForm based on this field
    did_vendor = models.CharField(max_length=10, default='unknown')
    did_active = models.BooleanField(default=True)