As it turns out, data-only plans are available on all four top U.S. carriers! Some are cheaper than others, but Sprint and AT&T have $30/mo data-only plans. For those who mainly rely on data connectivity with their smartphone, it can mean big savings while a Voice Over IP (VOIP) app can pick up the slack for making and receiving calls.

By leveraging Google Voice free SMS (text messaging) and Gizmo5′s VOIP technology, Android and Blackberry owners may have a very tempting offer right at their feet. For example, those who are looking for a capable device for their kids, but trying not to break the bank, may now have the perfect solution; a fun device with quality apps, unlimited texting and unlimited inbound/outbound calling.

With AT&T now allowing Skype to operate over 3G, iPhone owners can do the same by purchasing Skypeout minutes for their VOIP apps. Simply purchase a used or unsubsidized smartphone and call the carrier requesting service with data only.

VOIP apps are easy to come by, but finding one that works seamlessly with a single phone number, caller ID in both directions and with acceptable voice quality seems to be more difficult. After AT&T’s recent Skype-over-3G announcement, Verizon matched them to allow select Android and Blackberry devices to do the same. All that’s left is a follow-up announcement from Google regarding a G-VOIP app or Google Voice update to allow free VOIP calls, (trumping Skype’s paid Skypeout service), seamless inbound/outbound calling with caller ID and other advanced features. Surely, this solution will meld the efforts of the recently acquired Gizmo5 with Google Voice.

A brief history of each… in mid-2007, Google acquired GrandCentral, a feature-rich call forwarding service. Features were changed/added and it was re-branded Google Voice (GV). While it is not a true Voice Over IP solution, many assume it is. Calls placed and received using GV use a standard cellular telephone network, but are forwarded to/from Level3′s phone switches where GV’s rich calling features are implemented. GV is a valuable offering and does provide cost savings when coupled with certain cell plans*, but is still dependent upon the carriers voice network and an associated voice plan.

In late 2009, Google acquired Gizmo5, a San Diego-based VOIP startup that was once eyed by Skype. With a Gizmo5 account and a VOIP app such as Sipdroid (for Android), VOIP calls can be made and received outside of the carriers voice network using one’s Google Voice number. While Sipdroid is open sourced and works well, it may be difficult for the average user to configure and unfortunately, Google is not allowing new Gizmo5 signups so we have to wait for integration to occur.

A welcome solution will involve the VOIPizing of Google Voice, allowing users to bypass voice plans altogether if they wish, using a polished and seamless interface. As this transition to usable VOIP occurs, we should see per-minute and unlimited voice plans drop in price. In order to offset this, it would be reasonable to expect more customers will upgrade to smartphones with associated data plans.

* Verizon offers Friends and Family (F&F) plans that essentially allow unlimited outbound calling and inbound as well for those who wish to forgo inbound caller ID. By making the GV access number (916.538.3466) one of your F&F numbers, calls to and from this number will not count against the calling plan. Additionally, T-Mobile customers who still have ‘MyFaves’ can do the same, but these plans have been discontinued.

The main Gizmo5 site is currently down, but you can see what they were offering before the Google acquisition by visiting the dev site. Gizmo5 services included voice calls, instant messaging, file sharing, SMS, and video calls. Google is reporting the Gizmo5′s engineers will be joining the Google Voice team to continue improving the Google Voice experience.

I truly believe video calls from smartphones will be the next big thing and it could happen in 2010. The carriers are beefing up their networks (see T-Mobile’s HSPA 7.2) and will have the bandwidth to support such a service. The only thing we are missing are phones with front facing cameras and those are starting to appear.

Are we really ready for hand held video calls next year?

Features offered by Gizmo5.