FOUR out of five Irish mobile phone users are connected to the analogue network, recognised by its dialling prefix 088. The alternative system uses the 087 prefix and is called GSM (Global System for Mobiles). It uses digital rather than analogue signals.

GSM makes efficient use of the frequencies available for mobile communications and will eventually replace the analogue network, which is expected to be abandoned within the next decade.

GSM works by converting a conversation into a series of binary signals (like computer code) which are coded and then transmitted. The coding is what makes GSM calls impossible to scan, unlike calls made on the analogue network which are sent as a radio signal and which can be monitored by anyone with the right equipment.

One of the principal advantages of GSM phones is that customers can travel to any of the countries operating the system and still make or receive calls. Analysts predict the only two countries where GSM users will be unreachable will be the US and Japan.

All these advantages come at a cost to the GSM customer, however, who pays a 3p per minute premium for calls. GSM handsets are also considerably more expensive than analogue models, which can be bought for as little as £10.