To use the bomb, you will require another mobile phone in order to trigger the explosion from a distance. You sync the phone to the bomb by using the phone with Interact Key (Default F/Enter) while standing over the phone bomb on the floor, a message will appear saying the phone is synced.

Retreat to a safe distance and use the phone with Interact Key (Default F/Enter) to detonate the bomb.

For more information, visit the Explosives page.