How do you ensure all networks are represented and no respondent is interviewed twice in a poll?

Random interviews conducted in each poll cut across all networks in the proportion in which we have subscribers for them. According to Nigeria Communication Commission in December, 20181, the proportion for MTN subscribers is 38.9%, Globacom (26.2%), Airtel (25.6%) and 9-mobile (9.2%). Also, any Nigerian that is randomly selected in a particular poll from our database is not eligible to be called again until after six (6) months when he/she has an equal chance to be selected again among other Nigerians.