RTPEngine as you can find in the project GitHub too, is a proxy server for RTP and other UDP-based media traffic, and it is used with projects like Kamailio or OpenSIPS as a SIP proxy server. So, the SIP proxy handles SIP signaling, and RTPEngine takes care of the RTP/Media plane.

Over-The-Air (OTA) is a common concept that enables you to distribute various things, like a piece of code, to a device without physical access. In the context of mobile networks and SIM cards, OTA technology enables mobile network operators (MNOs) or MVNOs to change or update data in SIM cards deployed and get service without reissuing them! So, it means through remote provisioning, OTA reduces the cost of manual configuration not only for carriers but also for subscribers.