http://news.yahoo.com/s/ap/20071010/ap_on_hi_te/phantom_vibrations

the human body is a neuro-chemical factory. each experience we percieve is stored by chemical markers that match receptor sites in the brain.

when we visualise we re-stimulate the receptors by making these chemicals again and re-running the memory.

our emotions re-enforce this process and give us the sensation of cell-phones ringing when they are not.