[SR-Users] Kamailio IMS to IP PBX - how?

I'm trying to connect a VoLTE smartphone to an IP PBX using Kamailio IMS. I've managed to get the smartphone to connect to Kamailio IMS and make calls with our devices registered with Kamailio. The problem is that I need the smartphones to register with the IP PBX which is a VigorBX 2000n. This is so they can use an internal extension number and make calls to other local extensions with hard wired desk phones. Should I be using Kamailio IMS to "proxy" the smartphone SIP connection to the VigorBX PBX? Or would it be better to connect the smartphone directly to the VigorBX using SIP? If so, how? I've read absolutely every how-to document I could lay my hands on but none of them describe integrating IMS with a PBX. Thanks for any help / advice you can offer. Dave