Zte mf286d - rj11/ voip / sip

I plugged an analog phone into the RJ11 port on my ZTE MF286D running OpenWRT.
I get a dial tone and can call numbers, the phone rings when I call the number on the SIM so it's 95% working. But I can't find any references to configuring the voice service on the router.
Is there a config file somewhere or a menu that I've missed?

Thanks

There is no SIP client in this router, AFAIK.
RJ11 ports are wired directly to the modem.

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Ah, that makes sense. Thanks for the pointer.
I'll see if there's something in QMI or AT commands.