To trigger a phone call right from your EngagiFire popup or CaptiFire page, you can use the "Click-to-Call" component located at "Buttons" > "Click to Call"

Drag and drop it into the canvas, then from its settings, adjust the phone number option to the desired phone number.

On mobile, when your visitor taps/clicks on the button, it will automatically trigger a phone call to the specified number.

On desktop, the visitor will be prompted to choose which software to use to handle the phone call (Skype, Facetime... etc).

Depending on your campaign aims/audience, this may be a component you use on campaigns which are shown to mobile users only.

Further Reading:

Displaying Campaigns only for Mobile or Desktop

Click-to-call buttons