In Australia where line rental is expensive, it is common to have a 'Multiple Number' service, where the incoming ring patterns vary. (So a call to number 1 might sound like brring...brring..., but a call to number 2 might sound like brinng...brr...brr).
If it were on an FXS port, you'd be calling it distinctive ring, and talking about ring frequency and ring cadence - how the router generates these.
However, these parameters don't seem to apply to an FXO port where the router should detect these on a voice-port.
They should. Please add it to the product roadmap. Thanks.
PS. If it didn't work out well to have 'sub ports' of a voice-port, then you could at least have something like a 'ring mask' where the FXO port only answers if the detected ring-pattern was found. In that manner, you could electrically connect the same PSTN line to two FXO ports (although only one FXO port would ever seize the line), thus making your IOS development simpler, and selling more FXO ports ... although in practice, people with this setup won't be using more than 4 PSTN ports anyhow, in the main.
