Twilio, a cloud communications startup, has just closed a $12 million Series B.

This most recent round of funding was led by Bessemer Venture Partners and included Union Square Ventures (Fred Wilson's firm) and 500 Startups (a new firm founded by Dave McClure), as well as other prominent angels.

Twilio offers its clients pay-as-you-go communications services. It gives companies and their developers tools for making and receiving phone calls and communicating via text messages — all from computer programs and without human involvement.

Basically, Twilio helps other companies build web-based communications apps.

So far, the startup boasts around 20,000 customers — developers who've built apps ranging from web-integrated call centers to group-SMS and survey tools. Companies that use Twilio include Salesforce, eBay and Media Temple. You can see some of these companies and tools in the Twilio Gallery.

Here's a demo of a Twilio product that launched this year, OpenVBX for businesses:

Bessemer was formerly a Skype investor, as well; clearly, this firm has an eye on the future of communications.

Twilio will be using this new funding to expand its offerings and to hire sales and engineering talent — an increasingly competitive proposition in the current market in San Francisco.