One phone rings, then another, then four more, now a dozen. The 15th-floor conference room is suddenly abuzz with an eclectic mix of song snippets and audio bits, an intimate peek at their owners before each is picked up or silenced. Having impressed the audience with the telephony technology behind the product, the presenter moves on to the next demo.

The intersection of mobile and geolocation is still an unknown world, waiting to be invented by hackers like the ones at round 2.0 of TimesOpen, The New York Times’ outreach to developers, which launched Thursday night. We wrote about the first TimesOpen event last year: It’s an attempt to open the doors of the The Times to developers, technologists, designers, and entrepreneurs, who can use Times tools to help answer some of the field’s big questions. This iteration of TimesOpen is a five-event series this fall, each focusing on a different topic: mobile/geolocation, open government, the real-time web, “big data,” and finally a hack day in early December.

On the docket Thursday were Matt Kelly of Facebook, John Britton of Twilio, Mano Marks of Google, and John Keefe of WNYC. Kelly presented Facebook Places; Britton gave one of his now New York-famous live demos of the Twilio API; Marks dove deep into the various flavors of the Google Maps API; Keefe — the only non-programmer of the bunch — discussed lessons learned from a community engagement project with The Takeaway.

An API, or application programming interface, allows applications to easily communicate with one another. For example, any iPhone or Android application that pulls information from a web-based database is most likely it through an API. If you search local restaurants through Yelp, your location and query are passed to Yelp and results given in return. For any company with an API, like the three at TimesOpen, the challenge is to convince developers they should spend their time innovating on top of your platform. Strategically, when there’s an entire ecosystem living on top of your platform, your platform then becomes indispensable and valuable.

What’s most fascinating to me, however, are the approaches each company is taking to build a community around its API. The community is the most important key to the success of an API, a major source of innovation. One of the keys to Twitter’s explosive growth has been its API; rather than depending on its own developers for all new innovation, Twitter inadvertently created an entire ecosystem of value on top of their platform.

Let’s contrast Facebook and Twilio, for example. Facebook hopes Places, launched in mid August, will become the definitive platform for all location data. Interoperability can happen, but it should happen over Facebook’s infrastructure. Facebook envisions a future where, in addition to showing you where your friends are in real time, Places will also offer historical social context to location. Remember the trip through South America your friend was telling you about? Now you don’t have to, all of the relevant information is accessible through Places.

At the moment, though, Facebook’s only public location API is read-only. It can give a developer a single check-in, all check-ins for a given user, or check-in data for a given location. They have a closed beta for the write API with no definitive timeline for opening it publicly. Expanded access to the API is done through partnerships reserved for the select few.

Twilio, on the other hand, is a cloud-based telephony company which offers voice and SMS functionality as a service, and whose business depends wholly on extensive use of its API. Developer evangelist John Britton made a splash at the NY Tech Meetup when, in front of hundreds, he wrote a program and did a live demo that elegantly communicated the full scope of what their product offers. On Thursday, he impressed again: Using the Twilio API, he procured a phone number, and had everyone in the audience dial into it. When connected, callers were added to one of three conference rooms. Dialing into the party line also meant your phone number was logged, and the application could then follow up by calling you back. All of this was done with close to a dozen lines of code.

At TimesOpen, Britton stressed API providers need to keep a keen ear to their community. Community members often have ideas for how you can improve your service to solve the intermediate problems they have. For instance, up until a week ago, Twilio didn’t have the functionality to block phone numbers from repeatedly dialing in. For one company using the platform, the absence of this feature became a significant financial liability. Once rolled out, the feature made Twilio much more valuable of a service because the company could more closely tailor it to their needs. To make experimentation even easier, Twilio also has an open source product called OpenVBX and brings together its community with regular meetups.

Facebook already has the scale and the social graph to make any new API it produces a player. But for wooing the hackers — at least when you’re a small and growing platform — open and inclusive seems to win out over closed and exclusive.