If you have an unlimited cellphone plan and your bill never varies, autopay is a time-saving strategy for managing payments. There's also the option of setting up automatic payments for a non-unlimited cellphone plan. The problem, however, is that the amount you owe can change from month to month, depending on data usage. And if you forget to read your cellphone statement in months that you owe more than usual, the extra funds taken from your bank account could trigger an overdraft and bank fees.
Some cellphone plans also don't have the same billing date each month. While you might have a set date — say the 15th — others have a monthly cycle that varies depending on the number of days the month. That doesn't guarantee a consistent payment date each month, and you could be caught off guard — and out of funds — if a payment posts when you weren't expecting it.
Since autopay is a hands-off approach to paying bills, you're also less likely to inspect your cellphone bill after setting up automatic payments. That means you might not catch billing errors and will pay more than necessary.