How to remove 'ECO' on the telephone handset display.

It is not possible to remove.

This feature automatically reduces handset power consumption and extends the battery life by suppressing handset transmission power when the handset is close to the base unit. When this feature is activated, "ECO" is displayed.

Notes

For models with the "Link to cell" feature, during a cellular call, "ECO" is not displayed even though this feature is activated.
For models with Clarity booster/Sound enhancer, Eco mode is turned off when the Clarity booster/Sound enhancer is activated.
For Model with Answer by voice command, when Answer bu voice command is turned on, Eco mode is not displayed even though this feature is activated.