Why a small call test can pass
MTU limits the size of an IP packet on a link. VPN encapsulation reduces the space available for the inner packet. A short REGISTER can succeed while a larger INVITE with many headers or a larger SDP offer fails. This is a useful clue, not a diagnosis: authentication, routing and message-size limits can produce similar symptoms.
IPv4 fragmentation and IPv6 source fragmentation behave differently. IPv6 routers do not fragment forwarded packets. Missing fragments or blocked path-MTU feedback can leave the receiving application with no complete message.
Trace packet size at the failure
Record the transport, IP version, tunnel and effective path. Compare a working and failing transaction at both tunnel edges. Look for fragments, ICMP size feedback and retransmissions. A capture filter that selects only a transport port can miss later fragments that lack that header; choose a suitable host-level capture for this investigation.
SIP defines circumstances in which a request should use a congestion-controlled transport when it approaches the path MTU or is large with an unknown MTU. Verify that both peers actually support the chosen TCP or TLS transport before changing a trunk. TCP segmentation is different from relying on IP fragmentation.
Fix the path rather than shrinking everything
Correct tunnel MTU and permitted path-MTU feedback according to the network design. Avoid arbitrary global MTU changes without testing other applications. Reducing codec bitrate usually does not repair an oversized SIP request. Reducing packetization changes packet rate and delay as well as payload size. Retest the specific large message and the established media stream independently.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.