Voice has long been a prosperous kingdom ruled by operators, but times are changing in the mobile realm. No one is more aware than wireless carriers themselves. Over the past few years voice ARPU has been dropping at a disquieting pace of about 5 percent per year.

One way that operators have been fighting the declining revenue curve is by focusing their hopes on data and advanced services, but this alone is not enough. An additional response by operators has been to explore mobile advertising.

Currently, the most common channels used for advertising are SMS and the mobile Internet/WAP, which have been delivering promising results. However, more attention is now being turned to the vast arena of voice and the question being asked is, "Why not use the voice channel for advertising?"

The case for using voice as an advertising channel is compelling. Voice is still far and away the killer application, accounting for the lion's share of all mobile phone usage. It doesn't require smartphones, data or user education and it never becomes "uncool" or goes out of fashion.

As the most heavily used mobile service, voice calls offer the largest possible ad exposure in the mobile world and present a solid business opportunity.

A million users opting into a voice advertisement service would generate seven million RBT ad opportunities each day, assuming an average of seven calls per day, per user.

Even if an operator opts to use only a portion of the possible calls for ads, this still leaves several million new revenue opportunities every day from existing traffic, driving up ARPU without costing the users anything at all.

It is important that advertising on the voice channel is non-intrusive and does not interfere in any way with the actual conversation. Rather, advertising needs to be focused on other aspects of voice inventory -- the pre-call and post-call periods.

Instead of listening to system tones while waiting for a call to be answered, both parties can choose to listen to ad content. The caller can opt-in to listen and the called party can also opt-in to have ads played to his/her callers (users are motivated to opt-in because of incentives offered for participation).

To increase effectiveness, the ads can also be interactive. For example, the user can press "1" to skip or "2" to get more information via SMS after the call.

Due to the enormous volume of calls, the exposure potential is huge, and the attentiveness of the caller waiting for the call to be placed is high -- particularly because the rich store of information available to the operator about the user can be leveraged to target the ads to user interests.

A caller hanging up is not the end of the operator's revenue opportunities -- there is still money to be made.

Many operators offer complementary call services, for example notifying users by SMS of calls that were missed when the user was out of range, new voice message notifications, etc.

Advertisements can be inserted into call completion notification messages without requiring users to opt-in, as message alerts are offered for free by operators.

The reach of these ads is quite broad due to the fact these free services for call completion are usually auto-activated by operators to the entire subscriber base.
Even though use of the voice channel for advertising is a relatively new and unexplored realm, trailblazers in this arena have already been meeting with notable successes:

? Turkey: Winner of the GSMA Global Mobile Awards for 2009 in the category of best mobile advertising service was Turkcell, with its RBT Ad service Tonla Kazan (Tone and Win). Turkcell recently announced that the service reached 200,000 subscriptions in only eight months and has helped the operator to reduce churn.

? Czech Republic: Telefónica O2 explored a new concept of using ads in voice services by offering subscribers one minute of calling to any mobile or fixed number in the Czech Republic for free. This service was implemented by an IVR system, where users dial a special number and listen to a short commercial message.

They select the telephone number to call and receive one minute free of charge. By the end of 2008, more than 10 million calls were made in this manner, a very high number for a service launched only six months earlier.

? South Africa: Vodacom South Africa's "Please Call Me" service enables prepaid subscribers, who run out of credit, to send free SMS texts to a more affluent subscriber and ask to be called back. The advertiser pays for the SMS in return for inserting an ad, enabling advertisers to reach the mass market.

Vodacom reports that "Please Call Me" generates 20 million messages per day with 2.5 million unique users and additional revenues are generated when the calls are returned. Without the ad-sponsored service, this profitable activity would not take place.

Using voice for advertising benefits everyone. Subscribers gain the ability to enjoy discounted and free services and other incentives. For advertisers, the voice channel offers a broad, targeted and cost-effective reach to their target audience, with highly precise metrics. Operators gain a lucrative opportunity for a new revenue stream from voice services -- their greatest asset.

These are but some of the reasons that mobile advertising on the voice channel is well on its way to going mainstream.

Guy Yaniv is vice president and general manager of Comverse Mobile Advertising, Tel Aviv, Israel. Reach him at .