Attempted to use a phone on an exam, therefore received a "F" in a "Pass/Fail" Course
How long ago was this?
I can't sugar coat any of this, but frankly, this is a bit more serious in my own book. You made an overt attempt at cheating and you knew what you were doing. Plus, there's a lack of common sense in doing this.
I'm not ready to say that your medical career is over, but it might be in stasis for awhile.
You need to put as much time as possible between the incident and applying, and lead an exemplary life. No more screw-ups. Engage in lots of service to others less fortunate than yourself, and try to engage in positions of responsibility and trust, like teaching, or being a bank teller.
When it comes time to explain this, you have to own it. No excuses.
The problems that you face are that Adcoms will think:
1) is this the type of person we want in our class?
2) with all these other candidates who didn't cheat, why should we risk taking this person?
And no, this doesn't define you as a person, but yes, one stupid mistake can keep you out of medical school.