Step 1 – Navigate to Advanced > Sip Profiles
Step 2 – Click on Internal
Step 3 – Find variable multi-registrations contact false and change the value from false to true and click Save
Step 4 – Navigate to Status > Sip Status and restart internal service
Step 4 – Navigate to Applications > Ring Group > Select Your Ring Group > Strategy and change that to what ever that you have setup to Enterprise
Thats it, you can now register 1 extension on multiple endpoint devices i.e softphone or hardware phone etc.
One thing to note is that I am testing and implementing this on Fusion PBX 4.5.19. So, it may vary slightly based on what version you FusionPBX you are using.