FortiGate SIP ALG was silently breaking Zoom Phone registration

Yealink T54Ws were registering to Zoom fine but outbound calls dropped immediately after dialing. No errors in the Zoom portal — just silence.

Ran packet captures on the FortiGate and saw malformed SIP INVITEs leaving the firewall. Turns out SIP ALG was rewriting the SIP headers incorrectly.

Fix: Disabled SIP ALG under VoIP Profile and confirmed via CLI:

config voip profile
  edit "default"
    config sip
      set status disable
    end
  next
end

Calls stable immediately after. Classic FortiGate footgun — SIP ALG is enabled by default and almost always wrong.