Earlier we noted that traditional phone systems depended on the manufacturer of the PBX phone system to make the extensions that connected to it. With the introduction of the SIP protocol standards, any VoIP phone system that adheres to SIP standards can connect to any SIP-based extension that adheres to SIP standards. This is a powerful openness that the 3CX Phone System leverages to allow many phone handsets and softphones to be used as extensions with 3CX.
This wonderful ability for all SIP devices to interoperate is theoretical. Sometimes the SIP standard has multiple ways of doing something and different manufacturers use different methods making things break down. Thankfully, 3CX has done a lot of interoperability testing which is available at http://wiki.3cx.com/phone-configuration/firmwares-tested.
Let's take a look at some different categories of extensions that can connect to 3CX.