Ofcom has a range phone numbers they recommended for TV and radio drama purposes, read more on the Ofcom site.

Generate list of all mobile numbers


def print_mobile_numbers:
    for i in range(0, 1000):
        print(f'07700 900{str(i).zfill(3)}')
        
print_mobile_numbers()

Get a random mobile number


import random

def get_random_mobile_number():
    number = random.randint(0, 1000)
    return f'07700900{str(number).zfill(3)}'
    
mobile_number = get_random_mobile_number()
print(mobile_number)