This indicates an attempt to use the NVP-II protocol.
Network Voice Protocol (NVP) is a protocol for transporting human voice over packets in a network. It was one of the early VoIP technology. It includes several voice encoding techniques like linear predictive coding (LPC) and continuous variable slow delta modulation (CVSD). NVP consists of 2 parts: control protocol to initiate a session and data transport protocol to send packets.