The Total Access Communication System is now gradually being phased out in favour of digital cellular phone networks such as GSM and, in the future, UMTS.
It enjoyed a great deal of popularity in countries such as Italy and the United Kingdom in the 80's and early 90's.
One of its major disadvantages over digital networks is the absence of a SIM card: the phone serial number must be registered with the phone company for billing and caller identification purposes. Changing phone involves dealing with the company to use new new serial number.
Since this number is almost always stored in an EEPROM, it's relatively easy to change it, thus cloning the phone and making calls on someone else's bill.
Also, eavesdropping is very easy because of analogue signal transmission and the well know frequency of channels, around 900 MHz.