A quick grep on flags FL_* in the sources shows the following :
Flag 29 is used by acc module, 31 by nat_traversal, 0 to 12 in the parser.
Thus I assume that it is safe to test using flag 28.
I'll keep you posted on the test result.
You'll also find below warnings in msg_parser.h for the used flags. Since flag 30 is declared for mediaproxy in msg_parser.h, I'll change the flag of callcontrol to 28.
-----------------------------------------------------------------
/* WARNING: Value (1 << 29) is temporarily reserved for use in kamailio acc
* module (flag FL_REQ_UPSTREAM)! */
/* WARNING: Value (1 << 30) is temporarily reserved for use in kamailio
* media proxy module (flag FL_USE_MEDIA_PROXY)! */
/* WARNING: Value (1 << 31) is temporarily reserved for use in kamailio
* nat_traversal module (flag FL_DO_KEEPALIVE)! */
-----------------------------------------------------------------
$ grep -R 'define FL.* (1' src/kamailio/kamailio-3.2.0
modules_k/call_control/call_control.c:#define FL_USE_CALL_CONTROL (1<<30) // use call control for a dialog
modules_k/nat_traversal/nat_traversal.c:#define FL_DO_KEEPALIVE (1<<31)
modules_k/acc/acc.h:#define FL_REQ_UPSTREAM (1<<29)
parser/msg_parser.h:#define FL_FORCE_RPORT (1 << 0) /*!< force rport */
parser/msg_parser.h:#define FL_FORCE_ACTIVE (1 << 1) /*!< force active SDP */
parser/msg_parser.h:#define FL_SDP_IP_AFS (1 << 2) /*!< SDP IP rewritten */
parser/msg_parser.h:#define FL_SDP_PORT_AFS (1 << 3) /*!< SDP port rewritten */
parser/msg_parser.h:#define FL_SHM_CLONE (1 << 4) /*!< msg cloned in SHM as a single chunk */
parser/msg_parser.h:#define FL_TIMEOUT (1 << 5) /*!< message belongs to an "expired" branch
parser/msg_parser.h:#define FL_REPLIED (1 << 6) /*!< message branch received at least one reply
parser/msg_parser.h:#define FL_HASH_INDEX (1 << 7) /*!< msg->hash_index contains a valid value (tm use)*/
parser/msg_parser.h:#define FL_MTU_TCP_FB (1 << 8)
parser/msg_parser.h:#define FL_MTU_TLS_FB (1 << 9)
parser/msg_parser.h:#define FL_MTU_SCTP_FB (1 << 10)
parser/msg_parser.h:#define FL_ADD_LOCAL_RPORT (1 << 11) /*!< add 'rport' to local via hdr */
parser/msg_parser.h:#define FL_SDP_BODY (1 << 12) /*!< msg has SDP in body */
modules/mediaproxy/mediaproxy.c:#define FL_USE_MEDIA_PROXY (1<<30)
-----------------------------------------------------------------
Reda