some mysterious downloads everyday every 3 hours

Hi,

I have 4 iphones on $160 plan. We rarely use 4G data however in all 4 phones everyday every 3 hours some data gets downloaded.

On 3 phones, it is rather small but on one phone it is relatively high. For example, in 15 days one phone used up 1.8 gb whereas the other 3 used 0.3 gb.

I checked iphone settings and they seem to be right. Who is downloading this data everyday every 3 hours?

Is it ATT?

Regards,

Sankar