A Phoner is an awkward boner resulting from accidentally leaving your phone sitting on your junk. When someone subsequently calls, the vibration leads to a Phoner.
Bzzz bzzz bzzz.
John: Dude why the hell do you have a boner right now?
Alex: Someone called me three times and I was too lazy to move. Now I have a massive Phoner.
John: ....
When your phone lodges in your breeches at an awkward or uncomfortable angle, causing a suspicious bulge in the crotchal region. It does not necessarily mean your pants are too tight (a common misconception),and can usually be explained away or even pointed out for a laugh.