Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 108288)
+++ channels/chan_sip.c	(working copy)
@@ -2165,7 +2165,27 @@
 				if (sipdebug && option_debug > 3)
 					ast_log(LOG_DEBUG, "** SIP TIMER: Cancelling retransmit of packet (reply received) Retransid #%d\n", cur->retransid);
 			}
-			AST_SCHED_DEL(sched, cur->retransid);
+			/* This odd section is designed to thwart a 
+			 * race condition in the packet scheduler. There are
+			 * two conditions under which deleting the packet from the
+			 * scheduler can fail.
+			 *
+			 * 1. The packet has been removed from the scheduler because retransmission
+			 * is being attempted. The problem is that if the packet is currently attempting
+			 * retransmission and we are at this point in the code, then that MUST mean
+			 * that retrans_pkt is waiting on p's lock. Therefore we will relinquish the
+			 * lock temporarily to allow retransmission.
+			 *
+			 * 2. The packet has reached its maximum number of retransmissions and has
+			 * been permanently removed from the packet scheduler. If this is the case, then
+			 * the packet's retransid will be set to -1. The atomicity of the setting and checking
+			 * of the retransid to -1 is ensured since in both cases p's lock is held.
+			 */
+			while (cur->retransid > -1 && ast_sched_del(sched, cur->retransid)) {
+				ast_mutex_unlock(&p->lock);
+				usleep(1);
+				ast_mutex_lock(&p->lock);
+			}
 			free(cur);
 			break;
 		}