I am using baresip on my phone(from fdroid). I don't have my whole stack plumbed yet so I can't say how well it works. but it connects to my asterisk server via wifi well enough. I was expecting sip to be built into android but apparently it was removed in 10 or 11. but found the baresip package. I like baresip well enough for desktop use so I decided to try it on the phone.

It is one of those slow burn projects I work on every couple of months. mainly because I hate doing anything on the phone. but the idea is I have an asterisk server at the office that ties into the dialed phone world via sip trunk. I should be able to put a sip client on the phone, tunnel a secure connection to the office then the cell phone is just another office phone... anywhere in the world.

Only piece left is the tunnel, I have my heart set on ipsec. is this folly? should I be looking into other tunnel tech?