Internet monthly charge after cancelling service and returning equipment.

Did you return and cancel equipment within the allotted time allowed? Read your TOS, there are still charges you are responsible for when canceling within the buyers remorse period.

Equipment returned 24 April billing took place 28th April, and we have had the service at least 4 years so we should not have been billed on the 28th. We were instructed to return equipment by May 28th and complied well in advance as noted. We even had someone call and try to persuade us to return to ATT with a measly discount offer as soon as we returned the equipment.

billing took place 28th April

What (specifically) does this mean?

Your final bill is not prorated, if that is what you are getting at. 

No I have been charged for May and my account is still active, why

This is not a way to reach AT&T. This is a customer based public forum. What are the specific dates listed on your bill? From what to what? 

You posted this in the cell phone form. This should be in the internet form. 

Internet (and Cellular and TV etc) isn't canceled until you call and return equipment.  

Canceling and returning your equipment does not change your bill cycle.  It's a new bill was issued on the 28th, your bill cycle probably started over about a week earlier.  

And as stated, your bill cycle is not prorated when you quit service

When you say billing occurred on April 28th, do you mean that is the statement date, or the day a payment went through? Your cycle dates are what matter in determining if that was your last bill (which is not prorated) or if you were erroneously charged for the cycle that started after you called to cancel.

If you called AT&T to cancel on April 24th (or earlier) but have received a bill for service from April 28 thru May 27, that would be an issue. If that is for monthly service, and not any one-time charges to clear the account, then you will need to call AT&T to dispute. When you get their voice system say "cancel" to route you directly to the department you need.

Can only guess as to "why" your account would still be open. This is a publc forum, we cannot see your account.

Moving to Internet forum.