A new trend to avoid call charges - beeping - is frustrating telecoms in developing markets.

Imagine you want to communicate with someone on your mobile phone but your have little, or no, call credit. What do you do? The answer lies in "beeping" - also known as "flashing" - a phenomenon that is becoming ever more prevalent in the developing world.

While the price of a local call doesn't weigh heavily on the mind the average mobile phone user in affluent Ireland, the same can't be said for his or her counterpart in Africa, where mobile phones have become an essential - but at times costly - communications tool.

Hence "beeping", the practice of dialing someone's number and then hanging up moments later before the call is answered.

No words are exchanged, and critically no cost has been incurred by the phone user, but among regular beepers a specific message has been conveyed.

"Most beeps are requests to the mobile owner to call back immediately, but can also send a pre-negotiated instrumental message such as 'pick me up now', or send a relational sign, such as 'I'm thinking of you'," says Jonathan Donner, a researcher for Microsoft who has closely studied the phenomenon in Africa and Asia. "The practice itself is old, with roots in landline behaviors, but has grown tremendously, particularly in the developing world."

Donner, who has written a paper on the subject, to be published in this month's edition of the respected Journal of Computer Mediated Communication, first came across beeping while working in Rwanda.

He has since tracked it across Africa to southeast Asia and to India, where he is now based. Speaking to Innovation, he said studies show that roughly a quarter of all calls in Africa are "missed calls", and thus potentially beeps. A Kenyan mobile phone company recently estimated that it carried four million flashes per day. A separate survey found 38 per cent of public phone users in Uganda, Botswana and Ghana reported to regularly beeping mobile users from these telephones.

"These proportions are of significant concern to network operators since missed calls burden often-crowded mobile networks," says Donner.

Operators do benefit from the practice if the person who is beeped acts upon the signal by making a mobile phone call but this is by no means guaranteed.

Nor is it guaranteed, says Donner, that the practice will fizzle out as income levels rise in Africa and other such markets.

"Anecdotes from Africa and India tell of prosperous government ministers and captains of industry engaging in the practice. Apparently the head of India's Cellular Operators association and the chairman of one of India's largest telecommunication companies exchange (pre-negotiated) missed calls when they don't want to disturb each other."

Operators concerned about potential lost revenue from beeping have, in some instances, reacted by introducing alternative products, or restructuring their pricing regimes.

Some companies now allow their customers to send a limited number of free text messages when their call credit is low, while others have introduced low-cost SMS services for flash-type messages.

Orange Senegal, for example, lets customers send a "Rappelle-moi" ("Call me back") message when their phone credit drops below about 7 cent. In South Africa, Vodacom allows subscribers to send a "Please call me back" message for a fraction of the cost of a normal SMS.

As Donner puts it, "There is evidence that providers are trying to encourage alternatives to beeping within their networks to increase overall network traffic, but in ways that are less burdensome to the network than the missed call."

An interesting aspect to beeping is the way in which it has grown up by word of mouth rather than "by websites or manuals", Donner says. He cites a Ghanaian commentator describing it as "a distinctly national phenomenon", and recalls interviewing a Rwandan who began his explanation of beeping with the phrase, "You see, in Rwanda, we have a culture . . ."

The practice is known by indigenous terms - as "bipage" in French-speaking areas, and as placing a "miskin", or "pitiful" call in Ethiopia. But, despite such regional differences, there are common rules of engagement. According to Donner, these include:

• Never beep someone poorer than you. The general rule of "please call me back" flashing is that the richer person pays for the phone call.

• Do not beep too often. Donner says: "Beeping twice may increase the likelihood that a call target will call the beeper back" but any more than that and you run the risk of being labeled a nuisance flasher.

• Don't beep someone you are trying to impress. "If you are chasing a lady, you cannot beep. You have to call," said one Rwandan interviewed by Donner. "She has to see that the effort is being made. Borrow a friend's phone if you do not have airtime."