I have friends who left Russia. We mostly communicate through WhatsApp. Lately they've been asking me to make local calls to their utilities companies or whatever because those companies only accept regular phone calls and obviously making a regular phone call from a foreign country would cost them a fortune. I'm getting tired of playing this game of broken phone, spending hours on hold and arguing with bureaucrats.
If there a way I could set my phone in a way that would allow them to connect to it with voice over ip and make local calls from my sim this way?
They could get a local phone number in Russia from lots of VOIP providers, probably. Even with sanctions.
There's services like "mysms" which sync calls to their "cloud" service, but I'm not sure if you can place calls. That might work, worth investigating.
There's also these boxes that are sold as remote VOIP which accept SIM cards. They're marketed as "IKOS" online, can be found on Alibaba as well. I don't think they've made an LTE model, not sure where Russian cell carriers are in the lifecycle but most US ones have dropped 3G entirely.
There's some free ones but I don't know of any open source ones. Stuff like Microsoft Phone Link probably need you on the same LAN but you might be able to make it work using a VLAN VPN setup or something. https://support.microsoft.com/en-us/topic/setting-up-calls-in-the-phone-link-app-c7e75908-c65d-bd42-fcb2-ea4d5fb783f1
Someone who's used this feature says it works remotely:
But I still think the initial setup of Phone Link/"Your Phone" requires a LAN connection. Not sure how technically inclined you are but Wireguard and OpenVPN are both fairly easy to setup but I have no idea what ports this Phone Link thing uses for instance to really give a lot of advice. The closest I can think of are people setting up Wireguard on Windows for games with LAN multiplayer over the WAN. Basically the same thing, just more finicky. Here's the first result on the web when I looked for guidance on setting that up https://geekoverdose.wordpress.com/2020/04/18/play-lan-only-games-together-over-the-internet-with-wireguard/
Other ones worth investigating: iovox, Remote Phone Call (costs a few USD)
Thank you for the extensive write-up. I'll try to figure it out.