How to call a function from another function of sam class

Following is my code snipet. Please help me out:-

from sikuli import *
import comres # used for var inv_no

class phonevalidation:
    def blank_number(self):
        wait(1)
        click("1405933572316.png")
        while not exists("1405933601065.png",0):
            wait(1)
        click("1405933623586.png")

    def invalid_number(self,inv_no):
        wait(2)
        type(inv_no)
        wait(4)
        self.blank_number() #even tried phonevalidation.blank_number(). It is throwing error as "blank_number is not defined"

        while not exists("1405931822646.png",0):
            type(Key.BACKSPACE)

Calling from main:
phn=phonevalidation()
phn.blank_number()