https://carrier.huawei.com/en/technical-topics/wireless-network/VoLTE/2
Key VoLTE Technologies
Robust Header Compression (ROHC)
"ROHC compresses RTP/UDP/IP headers of voice packets and uses fewer fragmented packets to efficiently ensure the correct transmission of voice data packets and increase the cell edge coverage of voice services. ROHC uses different header compression algorithms for data flow based on different protocols. The ROHC compression efficiency varies based on the ROHC operating mode and changes of dynamic domains of packet headers at the application layer. Therefore, compressed packets appear in a variety of sizes. Packet headers can be compressed up to one byte, effectively reducing the size of voice data packets."
https://en.wikipedia.org/wiki/Robust_Header_Compression
In streaming applications, the overhead of IP, UDP, and RTP is 40 bytes for IPv4, or 60 bytes for IPv6. For VoIP, this corresponds to around 60% of the total amount of data sent. Such large overheads may be tolerable in local wired links where capacity is often not an issue, but are excessive for wide area networks and wireless systems where bandwidth is scarce.[1]
ROHC compresses these 40 bytes or 60 bytes of overhead typically into only one or three bytes, by placing a compressor before the link that has limited capacity, and a decompressor after that link. The compressor converts the large overhead to only a few bytes, while the decompressor does the opposite.