, the company that , has raised $19 million in a third round of venture capital to build an ambitious mobile platform for AT&T.

The platform, to launch in testing Nov. 14, will let the giant operator offer an array of Web services to users of its low-end phones — the majority of its phones, which don’t have the iPhone’s powerful features. The services will include things like social networking, email, RSS feeds and eventually services like Twitter.

It’s a gutsy move on the part of the iSkoot’s investors, who are increasing their bets at a time when most other investors are turning cautious because of the economic downturn. But it seems like a smart move for AT&T, which has already won big with the iPhone. That phone is helping AT&T steal a significant number of subscribers from other carriers. Other carriers are going to eye this latest deal carefully.

But Mark Jacobstein, chief executive of San Francisco’s iSkoot, while not commenting specifically on AT&T (we found out about it separately; the AT&T part is still under wraps) said such services are a natural extension of iSkoot’s technology.

So far, iSkoot has provided an application that allows cheap calls through the cellular voice network, which in turn is routed through iSkoot’s servers to their destination. ISkoot has been downloaded onto 300,000 phones. What is attractive to AT&T, however, is that iSkoot has managed to integrate the Skype application deeply into the phone software “stack” — namely in the form so-called “Skype phones,” where your Skype contacts are integrated right into your address book for example. If it can do that for Skype, it can do that for a range of other Web services — theoretically everything from Twitter to Facebook.

(While iSkoot also offers an application via its site for consumers to download directly to their phone, it’s iSkoot’s ability to integrate deep into the phone that apparently attracts AT&T).

The AT&T agreement will not be announced formally, but a test version of the product will be quietly circulated next Friday, VentureBeat has learned.

ISkoot is providing the service with help from extra technology provided by Social.IM, a social networking software that iSkoot bought in September. This is extra back-end help that will help iSkoot’s platform plug into various Web services. SocialIM was able to suck data out of Web services like Facebook, such as your contacts, and put them straight to your desktop in the form of instant messaging (IM) contacts. The immediacy of these web notifications are what iSkoot hopes to bring to the phone.

Jacobstein sees the SocialIM technology as a sort of “PointCast 2.0,” a reference to an iconic company that years ago tried to push data instantly to your PC. The technology needed to move data between the desktop and the phone, he says, is just as important as the technology that moves data from the Web to the phone.

ISkoot has done well relative to some other VoIP (Internet call) services because it is carrier friendly. Users still make a local call with its service (the local call is made to an exchange where it is then turned into an Internet call), which lets carriers at least make mone from those minutes. Many other VoIP services effectively let users circumvent carriers by going directly to the Internet cloud. It is also considered one of the top applications on the new Google phones using Android’s operating system.

If the iPhone and Google Android phones are bringing cool web services to users of iPhones, iSkoot sees itself as the way for carriers to bring services to low-end phones. There are few competitors offering software with such deep integration. Most of them are other messaging companies, such as Oz (bought by Nokia for a reported $260 million), Comverse, an IM client used by Samsung, Visto or Seven.

Investors include Vision Opportunity Master Fund, which led the round, and existing investors Charles River Ventures, Khosla Ventures, Jesselson Capital Corporation and ZG Ventures. The company has now raised more than $32 million.