The act of ringing someone's mobile (cellular) phone so that it rings just the once. This ensures that both the dialler and the diallee simultaneously now have the other person's number in their phones; and by hanging up before the conversation element of the call begins, any expense is negated.
This is used primarily in a face-to-face situation, such as when someone replaces their phone and needs to both re-acquire his friend's numbers and let them know his new number; or after two people meet for the first time and wish to exchange numbers.
"Mike, one-bell me so I get your number. Mine's 079..."