IUA, the SIGTRAN protocol that backhauls ISDN Q.921 signalling over IP — normally SCTP, with TCP and UDP registered too.
RFC 4233 defines IUA, which carries ISDN Q.921-user messages such as Q.931 call signalling between a Signaling Gateway and a Media Gateway Controller over IP. The RFC states the registered port for IUA is 9900, and IANA lists it on SCTP, TCP, and UDP; SCTP is the intended transport, so a TCP listener on 9900 is often something else entirely. You meet IUA inside telecom cores and in softswitch stacks bridging legacy ISDN to VoIP, not on general-purpose servers.
Do not expose
Telephony signalling between gateway elements, with no authentication of its own — it assumes a trusted operator network. Keep it on the signalling network.
$ sudo lsof -nP -iTCP:9900 -sTCP:LISTENSeen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.