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Every one of these is a real gap rather than a setting you have not found. They are listed so you can plan around them.

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An iPhone that is not open does not ring

This is the big one. iOS does not let an app hold a SIP connection while it is in the background. The sanctioned answer is a push that wakes the app a moment before the call arrives, and the phone half of that is built: the app obtains a VoIP push token and carries it in its SIP registration, so the server learns where to wake it from a registration it has already authenticated.
Not yet verified on a live call. Push wake-up is built and covered by tests, but nothing on the server sends the push yet, so no iPhone has been woken by one. FireTone's engineering tracker records this as an open check rather than a passing one. Treat what follows as the design, and expect rough edges the first time you run it.
Until that is deployed, an iPhone rings only while FireTone is in the foreground. Android is unaffected — it holds the line open with the background connection described in Installing it, which is on by default. See Mobile push for the server side.

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Video calls

Not in this build. The plumbing carries video and the account screen has a switch for it, but there is no in-call video UI and the path has never been run against a PBX — so every entry point is hidden and no camera permission is asked for. A number dialled as a video call is the one case that has no carrier fallback.

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Transfer, and merging two calls

Neither is in the app. You can hold a call, place a second, and swap between them, but you cannot join them or hand one to another extension. Both exist in the browser phone.

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Voicemail is a dial-through, not an inbox

There is no list of messages, no playback, no transcription. The Voicemail tab calls your mailbox and you listen to it as you would from any phone.

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Register on Wi-Fi only

The switch is on the account screen and the setting is saved, but nothing reads it yet — the app registers on whatever connection it has. Treat the switch as not yet doing anything.

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Self-signed certificates

A PBX serving a self-signed certificate over TLS cannot be trusted from the app; there is no switch for it. Use TCP against such a server, or install a certificate the phone will accept.

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UDP

Selectable, and it will register. It cannot carry a FireTone call — the INVITE fragments and the call fails every time. The app warns you in red when you pick it. Use TCP or TLS.

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On the iPhone only

  • No system call log. iOS opens it to nobody, so Recents holds FireTone’s own history. Calls placed from the Phone app are not in it.
  • No default-dialer role. iOS 18.2 added a Calling default in Settings, and the app will take you there. Below that version there is nothing to set.
  • No carrier voicemail number. Hence the voicemail extension setting.