Due to increasing demand on the current capacity of mobile phone networks, which frequently creates a saturation on individual networks, especially in densely populated areas, it has become necessary to implement multi-standard phones, capable of working on different networks with different frequencies and digital technology, thus permitting the subscriber to move freely within the areas covered by more networks.

A phone which works on two different networks can be classified as:

• dual band, when it uses the same technology, but different frequencies. For example, a phone that works on both a GSM 900 network and a DCS 1800 (also called GSM 1800 or PCN) network is called dual mode, because in this case both networks use the same transmission standard and the only difference between the two systems is the band that is used.
  
  

• dual mode, when it is able to connect to networks that use different technology (transmission standards and/or frequency bands used).An example of this would be phones that work both on land-based networks and on satellite-based networks.

An interesting phone is one which combines GSM and DECT technology, permitting the subscriber to "roam" and take advantage of the vast coverage of GSM service, when one strays from the city, and to utilize DECT service and all its advantages such as using the same number for home and office, PABX characteristics, excellent signal quality even inside buildings, and lower rates than mobile phone networks.

The commercialization of the first GSM dual-band mobile phones started after the CeBIT '97 fair, with some phones able to operate both on GSM 900 networks and on DCS 1800/PCS1900 networks. The PCS1900 is the standard adopted by the USA which uses the same technology as GSM, but on a frequency of 1900 MHz. Those who have such a phone (GSM dual-band GSM-PCS1900) are able to roam in the United States, all the while maintaining the same SIM card and phone number.

Another interesting characteristic is the dual band handover, which, for example, permits the user, who finds himself in an area covered both by the GSM 900 and by the DCS 1800 standards, to be able to transfer automatically from one system to the other in the middle of a conversation. Naturally, in this case, both the phone and the network must support this function.

In 1998, with the opening of satellite networks, dual mode mobile phones will come to market, that is, those which combine the technologies of satellite-GSM, satellite-DCS1800, satellite-PCS1900, satellite-AMPS, etc. These phones will be able to automatically transfer to a satellite network when there is no land-based signal coverage. In fact, even if the terrestrial cellular systems continue to grow rapidly, they will only be able to cover a small part of the planet's surface; as such, the new powerful mobile phone satellite networks will be a useful complement to those on Earth, facilitating communication in remote areas from anywhere in the world.