When signup is more easily managed by voice than by tapping into an online form, a click-to-call button is a perfect action for a mobile ad.

Although mobile ads typically have precious little real estate to work with, integration with the full functionality set of modern smartphones has enabled a broad range of actions that can extend what a user can do through interaction with the ad. We've seen examples of this in mobile applications. For instance, Urbanspoon was one of the first popular smartphone apps that encouraged you to shake your phone in order to serve up nearby restaurant suggestions, using the phone's accelerometer to detect a shaking motion.

Integration can also extend to popular native apps. Here are five actions that be linked to a range of user inputs (tap, shake, etc.) that leverage some of the most common usages of smartphones.  UPDATE: Set up a free Design Studio HTML5 account and build a interactive ads with the actions listed below yourself!:

1. Call to Phone Number. A very, very long time ago, before iOS and Android were battling for primacy, and even before the Blackberry, phones were used to call people. On occasion, they are used for them nowadays. And for certain cases, a voice call is indispensable when navigating through menus and tapping out a message is just impractical. Embedding a "click-to-call" button in an ad can reduce the friction a potential customer faces when electing to directly engage with an advertiser.

2. Send to Email. A phone's portability means that it's often used when we're on the run, and engagements can be short and sporadic. A "send-to-email" button embedded in an ad can forward information that a mobile user would like to review later, but simply doesn't have the time for at the moment they're viewing the ad.

3. Go to App URL. The Apple App Store and Google Play (Android's store for applications) are native apps that are incidentally the most popular apps, with a 99.5% and 96.5% reach among US users, respectively, according to the May 2013 ComScore Mobile Metrix. With more and more advertisers developing apps to provide users a more consistent, feature-rich gateway with which to engage with a brand, a button that takes a user to an app screen to download an app directly makes a lot of sense.

4. Add to Calendar. For event-based promotions, whether it's the release of a new album or flick, a streaming media event, or a sale, integrating a button that easily records a reminder for a user can be advertiser gold. The action can initiate a download of an iCalendar (.ics) file, which smartphones recognize and prompt the user to add the event to their default calendar app.

5. Share URL. When you have an item that's just too good to keep bottled in, providing a button that allows a user to tweet or share in social media can help drive viral spread. An ad update with breaking news, an out-of-the-world deal, or contests, for example, are perfect use cases for easily enabling shares in the fast-moving world of social media.

As we've said before, a simple clickthrough to an advertiser landing page or microsite should not be the goal for most advertisers. Providing opportunities for engagement with brand content, and giving interested users several options to make use of the content your ad highlights, can work even with the limited screen space of mobile devices.