Ask HN: Real-time availability calendars in online booking engines?

Many online booking engines struggle with one problem: real-time availability. Even well-established services like OpenTable are not immune to it. Ever been in a situation where OpenTable says there are no 7:30 pm reservations available, but when you call the restaurant, they say they can accommodate you? Businesses find it hard to rely solely on hosted availability calendars (esp. when it is provided by a growing startup). Today, they work around the problem by providing a fraction of their supply to online services while saving the remaining to make bookings in-house.

Any suggestions on how to make inroads and provide a reliable availability calendar that serves the needs of businesses as well as provides real-time availability information to online booking engines?