I mean FreePBX extensions can call Linksys SPA9000 extensions and otherwise.
The SPA9000 is limited to 16 extensions, so if you are trying to get passed that limit, why not just replace the SPA9000 with FreePBX and be gone with it? The SPA9000 is fairly rigid in what it can do for a purpose: Beyond 16 extensions, 1 IVR, etc. Linksys wants to move you up to a Cisco system.