Timer T399, in milliseconds, is the interval this port waits for an ADD PARTY ACKNOWLEDGE or ADD PARTY REJECT after sending ADD PARTY to bring a new participant into an existing multipoint SVC. This is the controlling timer for how long a multiparty call setup will wait on a single slow-to-respond leg before the switch gives up on adding that party, so it directly trades off call-setup patience against how quickly a failed add gets reported back to the application. An admin seeing elevated svcAtmConfigQ93bNumAddPtyRejPduRx counts alongside frequent T399 expirations would suspect a downstream switch or endpoint that can't keep up with add-party requests.