Set Fine: $400
Victim Surcharge: $85
Court Fee: $5
Total Fee: $490
HTA says that the penalty on conviction is from $300 to $1000 (maximum). So there's a possibility of the fine to be lowered below the OcJ "out of court fine" displayed on your Part I Offense Notice.
REG 366/09 Display Screens & Hand Held Devices: https://www.ontario.ca/laws/regulation/090366
Lesson to be learned here is to invest in a "car vent clip", "wind-shield mount" or "sticky-adhesive dashboard mount" and securely mount the phone to your vehicle.
Or keep your phone in your hand-bag/backpack/briefcase.
As long as you're not berating the officer and the courts; and you say your side of what took place. You're unshakeable under cross-examination, I still believe you will be found guilty.
Requirement that display screens be secure
8. "...device is placed securely in or mounted to the motor vehicle so that it does not move while the vehicle is in motion."
The courts will find the device was not secure. It was too accessible, and the display happened to be on during the course of the investigation. The perceived behaviour of texting from your lap and later tossing the phone to the passenger seat.
It's a very challenging charge for police to lay, especially for drivers who're texting with the device on their lap. By the time they're pulled over, the device will be far from their lap or seat.
I agree with @ynotp, that you shouldn't accept a guilty-plea especially if you're not-guilty. Share your side of the story, and let the police share theirs and let the Justice of Peace make a decision. Due to the nature of how the disclosure appears, it's not in your favour.
I don't believe the crown will be seeking a max $1000 fine, they're most likely going to ask for the fine set out on your Part I Offense Notice.