Picket fencing is slang for the chopping effect sometimes heard by cell phone users at the edge of a cell's coverage area, or (more likely) by the landline user to whom the cellphone is connected. "Picket fencing" refers to the way portions of speech are stripped from the conversation, as if the listener was walking by a picket fence

Picket fence

A picket fence is a variety of fence that has been used mostly for domestic boundaries. Until the introduction of advertising on fences in the 1980s, a Cricket field was also usually surrounded by a picket fence, giving rise to the expression rattling the pickets for a ball hit firmly into the...

, and hearing a conversation on the other side that changes audibily depending on the position of the pickets relative to the listener.

Originally from amateur radio

Amateur radio

Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...

, the phrase was used to describe the way an FM

FM broadcasting

FM broadcasting is a broadcasting technology pioneered by Edwin Howard Armstrong which uses frequency modulation to provide high-fidelity sound over broadcast radio. The term "FM band" describes the "frequency band in which FM is used for broadcasting"...

 transmitter will cut in and out as it nears the capture threshold

Capture effect

In telecommunication, the capture effect, or FM capture effect, is a phenomenon associated with FM reception in which only the stronger of two signals at, or near, the same frequency will be demodulated....

of the receiver, thus chopping the speech of the transmitting operator. It is not clear if the phrase was intended to describe the loss of the speech, or if it actually referred to the chopping sound itself, which imitates the noise produced by dragging a stiff object across a picket fence.