One of the supported gateways in 3cx for analogue line connection is the Linksys SPA3102. The setup of this to get it configured is very well documented here by 3cX but you may find that there is a little gotcha before that.
Plugging the wan port onto your network with result in a dhcp request – which you can normally check on your router to see. In my case I could see the address but could ping it only – not access the web interface. It took a bit of digging and a telephone handset to get the router ready for the integration with the 3CX switchboard. The problem is that access to the Web interface is disabled by default.
Complete these steps in order to enable WAN access to the SPA3102:
1. Connect an analog telephone to the PHONE 1 port of the SPA3102.
2. Pick up the handset and press the star key (*) four times: ****
3. After the greeting plays, dial 110# in order to retrieve the IP address of the SPA3102.
4. Complete these steps in order to enable WAN access to the SPA3102:
a. Dial 7932#.
b. Dial 1# to enable.
c. Dial 1 to save.
5. Hang up the handset.
Now access the Wan interface from a PC and change the address from dhcp to static. Once thats done you can set up the details to make the device accessible to 3cx as per the link above.