This seems like an issue with At&t in your area.
I am on verizon and on the rare occasion that I don't have 4g LTE service I only bounce down to 3G.
Which is part of the reason I left At&t.
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Isn't that when you are on a call?
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wow....glad I left At&t. Got sick of there crabby service.
Yes it happens during a call but it also happens randomly when I have poor signal.