I've played with doubango for some time trying to get it to work against the flavor of IMS used in VoLTE / VoWiFi, and it is not all that close to it, even with various hacks applied from https://gitea.osmocom.org/ims-volte-vowifi/doubango/commits/... in which I tried to add a Linux kernel IPsec plugin as well as various other bits and pieces.