LTE modem plus voice possible?

After exploring this forum and looking around at other forums of related topics, I am interested in exploring whether it's possible to use a mpcie LTE modem plus maybe something like Asterisk to use as a voice gateway for my network, in addition to using it as a data/sms router.

I got the Quectel EG25-G mpcie modem to put in my APU3. After reading around it seems like something like this might be possible. Has anyone done something like this before or have any further reading I might be able to pursue?

I was reading a little about UAC (USB Audio Class) on this modem, but it doesn't look like it's enabled. I wasn't sure if that's the direction to go.

I know I can just use VOIP with a SIP service, but that's not really what I'm looking for. I also realize that VoLTE might be a way to go, but I can't imagine that there's a VoLTE client that would be made available by a provider (which is where I imagine those come from) that would work on OpenWRT or normal Linux.

See this rant regarding UAC:

Still valid AFAIK.

But you might be lucky. Did you see and test this?:

The Quectel docs are linked from the forum thread in that discussion.

With regard to VoLTE I believe that is often implemented in modem firmware as well, so the host will simply deal with it as an ordinary audio call. But again you're depending on firmware features which might be disabled. Or requiring poorly documented modem specific magic commands:

With regard to VoLTE, please double-check with your mobile operator. Here in the Philippines with Smart, this feature is only available on postpaid plans (not prepaid), and only with a few whitelisted models of phones, and there is no way to get them whitelist your non-standard device even if it works with another operator.

Thanks, I'll look into that.

Same in the UK. Operators whitelist mobile phones, not routers. But are such operators actually able to verify the IMEI that is presented by a client device (mindful that this can be manipulated in many devices)?