Have you manually provisioned the device using the information from the extension you created in 3CX?
Have you checked the 3CX Activity log, to see if it is receiving registration attempts from the device? Do you have it set to use STUN? Are there other SIP devices at that location?
have a read through...
https://www.3cx.com/community/threads/3cx-and-cisco-spa112.53696/
https://www.3cx.com/community/threads/linksys-pap2-ans-3cx-compatibility.47907/
https://www.3cx.com/community/threads/linksys-spa2102-and-3cx-in-cloud.67009/
As i said, most Cisco/linksys devices are
very similar, in particular the basic settings, required to get calls going in and out. The configuration of your local network, other local devices, etc. , can affect how well this will work. If you are using STUN, then the local port, on each line, must be unique to the entire local network. No conflict with any other SIP devices, that already exist at that site. It is different if you are using an SBC.