class Company(models.Model):
    name = models.CharField(max_length=250)
    ...
class Contact(models.Model):
    company = models.ForeignKey(Company)
    phone = models.CharField(max_length=100)
    ...

company = Company()
company.name = 'firma'
company.save()
contact = Contact()
contact.company = company
contact.phone = '12345679'
contact.save()

company = Company()
company.name = 'firma'
company.save()
contact = Contact()
contact.company = company.id
contact.phone = '12345679'
contact.save()

company = Company()
company.name = 'firma'
company.save()
company.contract_set.create(phone='12345679')