"Please Call Me" messages are a popular cultural form of mobile communication in South African society. These USSD messages are used to advertise everything from car insurance to domestic airlines to the local corner store. Today, thanks to a recent project by SocialTxt, these free messages are used for the first time ever for social marketing: to encourage South Africans to get tested for HIV and obtain AIDS information.

"Please Call Me" (or PCM) messages evolved from the practice of "beeping," or calling someone and hanging up after a ring or two. Beeps, also known as missed calls or flashes, are a signal for the recipient to call someone back when the caller is low on airtime. Carriers started providing PCM messages free of charge after they found that the networks were getting inundated by millions of beeps a day. Studies estimate that between 20 and 30 percent of calls made each day in Africa are beeps.

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