The first cordless phones were introduced to the public around 1980. They operated in the 27 MHz frequency range, which is the same frequency range used by CB radio. There were only 10 channels and each phone was given one of those ten. If your neighbor had a cordless phone that used the same frequency as your cordless, you could end up with a 3 or 4-way conversation.
If you unplugged the base for your cordless phone it would find the base of your neighbors's cordless phone, provided they shared the same frequency and your neighbor was doing something else. You could then make long distance phone calls that would be billed to your neighbor. Most people exchanged their cordless for one with a different frequency.
In 1986 the FCC opened up a new frequency range for cordless phones in the 46-49 MHz range. There were still only ten channels but there was less noise on the higher frequency. Power was reduced to help eliminate unwanted "conference" calls and the potential for long distance phone call theft. However with less power most phones had a limited range.
Anyone who still uses one of these knows that when you move through your living space the signal may come and go. By 1990 there was a lot of crowding in this frequency range so the FCC released the 900 MHz range for cordless phones. This higher frequency was in the UHF range and provided a much cleaner signal. There were about 100 channels available too. The newer phones were very expensive, starting at about $400.00. All of these phones were still susceptible to eavesdropping by anyone wih a scanner radio. One way to reduce this was to transmit a digital signal.
In 1994 the first digital cordless phones were sold. The previous phones used analog technology which used normal voice for the signal. The digital signal was more difficult to decipher. In 1995 digital spread spectrum was incorporated into cordless phone technology. This made eavesdropping nearly impossible since the signal was digitally spread out across the frequency range. The signal was only on one given frequency for a very short time before it hopped to another. More power was added, up to one watt, which is lot for a cordless phone but it drained the batteries in a rush.
Caller ID was introduced on cordless phones in 1996. As prices began to come down some large families wanted more than one cordless phone in the home so two line cordless phones were developed. As technology continued to improve prices continued to fall. More people could afford the more advanced phones. In 1998 the FCC granted the 2.4 GHz cordless phone spectrum. This increased the range from the phone to the base to almost 2000 ft.; over 1/4 of a mile. It was also above the frequency range of most scanners.
In the year 2000 DSS cordless phones were selling for less than $100.00. The FCC opened up the 5.8 GHz range in 2001 and in 2002 the first 5.8 GHz cordless phones went on the market. The highest frequency that some of the most expensive receivers can receive is 3 GHz. This has increased the security of eavesdropping even more. Only Uncle Sam and some electronic gurus could listen to your cordless signal via the airwaves at this frequency.
Cordless Phone History (www.affordablephones.net/HistoryCordless.htm)
Cordless Phones (inventors.about.com/library/inventors/bltelephones5.htm)