So an AT&T and Verizon M8 are both getting random static interference about one a week during calls.
Is this in a specific place? LIke work or home? is it happening in a large geographic area or have you noticed this in one spot, like work or home.
I ask because you have ruled out a carrier issue. Since its happening to 2 different carriers on the same device, this would make me think outside interference. Since your hearing the static the inbound traffic basically is being interfered with so bad that your hearing the device try to compensate so hard its allowing a bad signal in, which is very odd across two devices.
Thats my best guess. sorry i cant help more, had it been on just one carrier it might have been a handset or tower issue, but with it being on 2 different carriers that makes me think its what i described as above.