Research was performed by Dr. Patricia Rusin,  Dr. Charles Gerba and Sheri Maxwell at the University of Tucson, Arizona . The work was funded by Procter and Gamble. 

Large numbers of Salmonella (a common bacterial cause of diarrhea) may be found in the stool of an infected person. Hence.   If even only a tiny amount of stool were transferred from an infected person's contaminated hand to a telephone receiver, the next user could have 107,104 Salmonella cells on the fingertip. If an inficted area were  placed in the person's mouth, the person would receive a dose of 36,383 cells which could easily result in infection.

Studies show that viruses can survive for hours to days on a hard surface.   Infectious are often very low and large numbers are often found in human fluids such as nasal mucus and stools.  if a virus such as the rotavirus (a diarrheal virus) were on the surface of a telephone receiver, infections could easily be transferred to people via telephone.  For example, if  a telephone receiver were contaminated with a low concentration of rotavirus agent (e.g. 10,000 viruses) 6,580 of these would be transferred to the hand during normal use of the telephone with 211 of them found on the fingertip.   The results show that 72 viruses could be ingested by the user which would very likely result in disease.