As per the link some Cisco legacy phones are supported with 3CX Phone system. These models are the:
Cisco 7900 Series: 7940, 7941, 7960, 7961
Cisco SPA Series: 302, 303, 501G, 502G, 504G, 508G, 509G, 512G, 514G, 525G/G2
Limitations of Legacy Phones
Cisco phones sit in the legacy phones “supported” section (but not preferred) what this means is that you will not get the full features and functionality that 3CX Phone System can provide. I have outlined some of these limitations below.
Provisioning:
Plug and play provisioning is not supported with Cisco phones – both for internal PnP and across the 3CX SBC. You can manually set them up but this is not recommended.
You will need to configure the phones either using DHCP Option 66:
http://www.3cx.com/sip-phones/dhcp-option-66/ or if you do not have a server available to facilitate Option 66 then you will need to manually configure or use the alternate provisioning option in the below link/guide:
http://www.3cx.com/sip-phones/cisco-spa501g/
CTI Mode with 3CX Softphone:
Cisco phones do not work in CTI mode with 3CX Phone for Windows:
http://www.3cx.com/user-manual/installation-windows/
Limited "make call" functionality – System see’s the phone/Softphone as 2 different entities.
Hosting & Remote extensions
Various issues with Hosted (VPN with DHCP option 66 seems the only solid way to get them working) - no STUN no SBC:
http://www.3cx.com/forums/post174313.html?hilit=Cisco#p174313
http://www.3cx.com/forums/post167034.html?hilit=cisco hosted#p167034
All the information on Cisco phones with 3CX can be found here:
http://www.3cx.com/blog/tag/cisco/