The modem can attach to the IMS APNs, however you'll need to do SIM-specific authentication which requires being able to send the right APDUs to the SIM. Some modems expose the SIM card in one way or another (either an AT command to send APDUs and get responses or as an actual USB smartcard reader) and it would be possible with those to do the authentication flow and then register onto the IMS server (which speaks SIP).
Alternatively, if you're happy with circuit-switched calls, some modems will expose the raw audio (as PCM data) as a separate serial port from which you read/write, and control the call flow using AT commands on the main serial port. Some modems need this to be first enabled via a poorly-documented AT command, and some might need authentication (security by obscurity) to do so which software such as DC Unlocker can break.
In addition to the EAP-AKA' authentication, the strange and highly opaque ways through which VoWiFi support is signalled and enabled might be an issue here too.
I've seen brand new Pixel devices (which fully support VoWifi) act like they have never heard of the feature, until a special cell broadcast SMS is received from the carrier, to turn the feature on.
It would be interesting to see if you could manually tick all the boxes and open up a tunnel to the ePDG etc without that happening, but given the number of moving parts, and the differences between each network's implementation, I'd be sceptical it would work reliably.
(I'm pretty sure I had a situation where individual batches of IMEIs were being whitelisted for WiFi calling on another network, where the switch appeared, but VoWiFi didn't work... Yet on the Nexus device in question, one bought from the carrier directly did work. Even when you manually flashed both devices with identical firmwares from Google directly, this remained the case - must have been an IMEI whitelist or similar).
Ah I see, so I did find someone that tried to do this here [1], but it seems like they were not even able to connect to the SIP server. Bummer, I wonder if someone else has had success.
