For every dollar the phone companies make on data services, they used to make two to four times that on voice services. That is, until now.

In 2010, the average revenue per user (or ARPU) for wireless consumers decreased $0.46. Within this revenue shortfall, voice ARPU fell the most - $2.86 per user. Meanwhile, data ARPU actually increased by $2.37. That means phone call revenue is falling by the wayside, and data and apps are the new trend.

To the average person, this flip-flop in mobile operator revenues probably doesn't seem like that big of a deal. Just replacing one revenue stream with another, right? Wrong. When this happens, it will spark a crisis among the major wireless providers like AT&T, Verizon and Sprint.

Here's why:

1. Voice plans have a higher price threshold among consumers than data plans. So, bottom line: Phone companies can charge more money.
2. Mobile operators have a fortified position in voice, but the data market is up for grabs. There are too many players in the data market (large companies, small companies, startups) that can set up solutions quickly and become instantly competitive.
3. The existential threat: If voice service dips too low, could it become irrelevant? That might sound crazy, but recent studies have found a 700 percent increase in texting over voice calls - a trend that seems only to be strengthening now that social media is being embedded in the user-interface designs of the latest smartphones.

In spite of all the glitz and glam of mobile data, it is the traditional phone call that mobile operators still depend on for making their billions in annual revenues. Data gets all the hype as the new consumable, but mobile operators can't afford to let the voice market erode. After all, voice is the one place where operators still hold all the cards.

The questions mobile operators need to be asking themselves are: Why are consumers turning to data instead of voice? How can they incentivize voice services again? How can they reduce the instances of missed calls to keep users connected?

There are a number of ways to answer these questions, but here are the top five ways mobile operators can overcome the "voice" slump:

1. Aggressive Voicemail:
One of the biggest challenges to voice service is uncompleted calls. When a call is made but not answered, that billing opportunity dies with it. Voicemail is considered a "black hole" by operators. No one wants a call to end up there. If mobile operators can increase call completes by just 5 percent or 10 percent, they boost their annual revenues by eight or nine figures.

The trick for operators is to re-engineer voicemail to become a find-me service, rather than a repository of dead calls. For one thing, operators can do more to alert users when they miss a call. These new alert and notification methods should involve a combination of SMS, chat, email and Web 2.0 services. Forwarding missed calls via "intelligent routing" is also key. It means voicemails will try to find the user, going from one device to another, until they are able to connect. So a missed work call on a BlackBerry might track the user down on a personal iPhone, iPad or even Roku or Wii.

2. 4G video calls:
We don't have these yet, but we should. The popularity of video chatting, whether it's via Google Talk, Apple's iChat or Facetime, Skype or Twilio, hasn't gone unnoticed by the mobile operators. For one thing, these are the types of calls that consumers actually want to make - particularly the younger age markets (13-17, 18-24 year olds). And whether the call is being made from a Skype app on an Android phone, or Facetime on the iPhone, the calls aren't billable to the phone companies. That needs to change. As soon as phone companies can offer video chat features via the traditional 3G or 4G channel, it will incentivize calls.

3. Better phone directories:
When phone companies make it easier for users to import their address books from Gmail, Yahoo, Apple, Skype, etc., they will have an easier time making the call. Phone companies need to get behind new features that allow importing email contacts, address books, etc., into a phone directory easy and seamless.

4. Messaging within the call:
Given the integration of calling, data and Web 2.0, enhanced phone calls is a natural next step. Allowing a caller to input a message or link into the call would greatly enhance the likelihood of pickup on the other end. For example, when calling a friend or co-worker, being able to have a message appear inside the caller ID ("This is about tomorrow's report" or "Extra ticket for this weekend") would increase the chances of the person on the other end actually answering the call.

5. Cheaper international rates:
Mobile operators must also cut back on their international call rates. They are already getting hammered by alternatives like Skype, Vonage and the many Web 2.0 chat options, and there comes a point where there's nothing to do but scale back the rates. Given the decline in overall phone calls and the ease with which consumers can bypass expensive overseas calls by resorting to VoIP or Web 2.0 services, it's a no-brainer that phone companies will have to compete by making these types of calls cheaper.

Eric Chan is president of Mobileslate.com