Agree with 4ymm... you can have both 401k & 401a in retirement deductions. I've done it.
In this case, 401a stands for after-tax. 401k is pretax contributions. Once you hit the IRS limit for pre-tax dollars, anything exceeding is taken as after-tax dollars. You paid the taxes on it in your paystub first. The benefit is that anything contributed after-tax grows tax free.
If you dont want to contribute after-tax once you hit the IRS $19k limit, you have to turn that off on Fidelity's website. Or call them up.
If you purposely contribute both pre & after tax contributions all year long, then you can manage on the Fidelity website too.
Finally, AT&T only gives a company match if you make a contribution. Pre-tax, after-tax...doesnt matter. If your contributions stop, then there is nothing to match, and you lose out on free money. So contribution timing is key to maximum savings. Good luck.