class Group(models.Model):
    name = models.CharField(max_length=30)

class Person(models.Model):
    group = models.ForeignKey(Group)
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30)

class Phone(models.Model):
    persons = models.ManyToManyField(Person)
    number = models.CharField(max_length=30)

class PersonInline(admin.StackedInline):
    model = Person

class PhoneInline(admin.StackedInline):
    model = Phone # also tried: Phone.persons.through

@admin.register(Group)
class GroupAdmin(admin.ModelAdmin):
    inlines = [PersonInline, PhoneInline]