code: func (m *Manager) SavePhone(username string, phonenumber Phonenumber) error {
  	if err := m.RemovePhone(username, phonenumber.Label); err != nil {
  		return err
  	}
  	return m.getUserCollection().Update(
  		bson.M{"username": username},
  		bson.M{"$push": bson.M{"phonenumbers": phonenumber}})
  }
docs: // SavePhone save or update phone along with its label