BSSGP SUSPEND ACK with unknown BVCI=0
When a GPRS-attached MS initiates a circuit-switched service (e.g. SS/USSD or SMS), OsmoSGSN logs the following:
DBSSGP NOTICE gprs_bssgp.c:556 BSSGP BVCI=0 Rx BVC STATUS, cause=Unknown BVCI DBSSGP ERROR gprs_bssgp.c:563 BSSGP BVCI=0 Rx STATUS cause=Unknown BVCI missing conditional BVCI IE
As far as I understand, OsmoPCU sends BSSGP SUSPEND (BVCI=0) to OsmoSGSN, and gets BSSGP SUSPEND ACK (BVCI=0) from it. So then OsmoPCU realizes that BVCI=0 is now known for some reason, and sends BSSGP STATUS (BVCI=0) with cause "BVCI unknown (5)".
Please see an attached capture. OsmoSGSN is bound to 127.0.0.10:23000, OsmoPCU is at 127.0.0.1:23000.