Abstract: In connection with enhanced telephone service, such as personal number service, the detection of whether an incoming telephone call is a facsimile transmission or an ordinary voice call is improved by a utilizing a modified ring back signal during the time in which the service circuit node of the telephone system is attempting to detect the presence of the calling tone (CNG signal) of the sending facsimile machine. Particularly, the ring back signal is modified to increase the off time of the ring back signal to be greater than the period of the CNG signal. Where the CNG signal is a periodic signal with a period of 4 seconds with a 0.5 on time and a 3.5 off time, the modified ring back signal has a period of 7 seconds with a 2 second on time and a 5 second off time.