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With the push of a button — and at the direction of President Donald Trump — an alert will be sent to cellphones across America at 2:18 p.m. ET on Wednesday: "THIS IS A TEST."

The message is scheduled to be the first trial of the "Presidential Alert" system, a new way to notify Americans across the country of national emergencies.

The Federal Emergency Management Agency oversees the Wireless Emergency Alert (WEA) system, which has sent out 40,000 messages to cellphones since its launch in 2012, warning people of everything from local severe weather to Amber Alerts.

The "Presidential Alert" is a new type of WEA notification meant to be used only for national emergencies. Unlike other alerts, people with cellphones cannot opt out of the messages, theoretically giving the president a direct line to the mobile devices of around 95 percent of Americans.

The wireless alert test at 2:18 p.m. ET will be followed by a test of the Emergency Alert System (EAS) at 2:20 p.m. ET on television and radio.

The only way for U.S. cellphone users to not get the "Presidential Alert" is to have their phone switched off, not be within range of an active cell tower, or be with a wireless provider not participating in the WEA.

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FEMA and the Federal Communications Commission currently have agreements with more than 100 cellular providers who voluntarily participate in the WEA system to ensure their users receive the alert.

FEMA initially announced that they system would be tested on Sept. 20, but later pushed it back to Oct. 3, citing the "ongoing response efforts to Hurricane Florence."

Reports of the new alert received a flurry of online backlash as many Americans worried the messaging system would be used inappropriately by the president, similar to his Twitter feed.

In 2015, Congress enacted the Integrated Public Alert and Warning System Modernization Act to limit the scope of what can be considered a valid emergency alert.

"Except to the extent necessary for testing the public alert and warning system, the public alert and warning system shall not be used to transmit a message that does not relate to a natural disaster, act of terrorism, or other man-made disaster or threat to public safety," the act states.

Don Hall, the government solutions director for OnSolve, an emergency notification service that works locally with FEMA, said the system is powerful because of its reach and immediacy.

"The [wireless emergency alert] is pretty much sent instantaneously," Hall said.

According to Hall, there is not much room for a personal message from the president within the alert's current 90-character limitations.

"There are plans for the FCC to extend the character count to 360 in May 2019," Hall said. "But as of now, you can't say a whole lot with 90 characters."

In the rare instance of a "Presidential Alert," the message will state the type of emergency and advisory directions to follow.

"If the 'Presidential Alert' is used correctly, it will be very effective in helping save lives," Hall said.