Are there any manufactures of IP based Audio Door Entry Systems (Intercom System) that operate / integrate with Ring Central
cyberdata.net would be a good bet and they have good support and know RingCentral. Really any SIP device should be able to work with a little effort and any analog device can work with an ATA. The analog option is often good if you can reuse cable or existing equipment and pulling ethernet turns out difficult/expensive.
Direct link to their door entry/intercom products: http://www.cyberdata.net/products/voip/digitalanalog/intercom_endpoints.html
We use Algo devices and they work great http://www.algosolutions.com/products/doorphones-security/8028.html