Plugging an electrical device into a power plug is the ultimate experience of compatibility. But as with international standards for power plugs, you must take care about interoperability between SIP devices.
pbxnsip makes interoperability as easy as possible:

• The PBX uses the "back to back user agent" (B2BUA). This has the advantage that the interoperability complexity is dramatically reduced. Devices only have to be compatible to the PBX, but not to all other devices that are connected to the PBX. For many SIP trunking providers this is a must-have in order to receive their service.
• pbxnsip was written by people that understand SIP. The call flows are simple; they make it easy for connected devices to be compatible to the pbxnsip PBX. The PBX does not attempt to offer a large number of protocols, instead it focus one protocol and does that protocol right.
• Critical topics that vary significantly between the different SIP implementations can be configured. For example, the indication of a specific ring tone to the SIP phone can be configured in a XML file.
• For most popular partners, pbxnsip performs third-party interoperability tests. This gives you additional security that buying SIP gear is a safe investment.

The PBX also supports the "plug and play" experience for a list of selected devices. The PBX generated the necessary configuration files on the fly. Check out the Wiki for the list of currently supported devices.