A man walks with an umbrella outside of AT&T corporate headquarters on March 13, 2020 in Dallas, Texas.Ronald Martinez | Getty ImagesAT&T shares fell Thursday after the company said its cash flow was hurt by customers' later phone payments and company spending on building 5G infrastructure.The company said customers have been paying their bills about two days later than they did the same time last year. That alone affected about $1 billion in quarterly cash flow, the company said."There's clearly some dynamics in the economy. We have customers that are stretching out their payments a little bit," AT&T CEO John......