SIP and RTP signalling compression is defined in SIGCOMP, RFC 3320.
Another, more minor, way of compression is the compact headers, defined in various RFC:s.
About this in the SIP RFC 3261:
7.3.3 Compact Form
SIP provides a mechanism to represent common header field names in an abbreviated form. This may be useful when messages would otherwise become too large to be carried on the transport available to it (exceeding the maximum transmission unit (MTU) when using UDP, for example). These compact forms are defined in Section 20. A compact form MAY be substituted for the longer form of a header field name at any time without changing the semantics of the message. A header field name MAY appear in both long and short forms within the same message. Implementations MUST accept both the long and short forms of each header name.
Example
Content-Encoding: gzip
e: tar
April 7, 2023
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