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)}')

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()