from django.core.validators import RegexValidator
class PhoneModel(models.Model):
...
phone_regex = RegexValidator(regex=r'^+?1?d{9,15}$', message="Phone number must be entered in the format: '+999999999'. Up to 15 digits allowed.")
phone_number = models.CharField(validators=[phone_regex], max_length=17, blank=True) # validators should be a list