--- channels/chan_sip.c	(Asterisk 13.5.0)
+++ channels/chan_sip.c	(working copy)
@@ -3952,14 +3952,13 @@
 					sip_methods[pkt->method].text,
 					pkt->method);
 			}
-			if (!pkt->timer_a) {
-				pkt->timer_a = 2 ;
-			} else {
-				pkt->timer_a = 2 * pkt->timer_a;
+
+			siptimer_a = pkt->timer_t1 < pkt->retrans;	/* Double each time */
+			if (siptimer_a < pkt->timer_t1 * 2) { /* signed int overflow? */
+				siptimer_a = INT_MAX; /* Workaround any overflow, like negative value or 0 */
 			}
 
 			/* For non-invites, a maximum of 4 secs */
-			siptimer_a = pkt->timer_t1 * pkt->timer_a;	/* Double each time */
 			if (pkt->method != SIP_INVITE && siptimer_a > 4000) {
 				siptimer_a = 4000;
 			}
--- channels/sip/include/sip.h	(Asterisk 13.5.0)
+++ channels/sip/include/sip.h	(working copy)
@@ -1223,7 +1223,7 @@
 	int response_code;        /*!< If this is a response, the response code */
 	struct sip_pvt *owner;    /*!< Owner AST call */
 	int retransid;            /*!< Retransmission ID */
-	int timer_a;              /*!< SIP timer A, retransmission timer */
+	int timer_a;              /*!< not used anymore, re-using retrans now */
 	int timer_t1;             /*!< SIP Timer T1, estimated RTT or 500 ms */
 	struct timeval time_sent;  /*!< When pkt was sent */
 	int64_t retrans_stop_time; /*!< Time in ms after 'now' that retransmission must stop */
--- main/sched.c	(Asterisk 13.5.0)
+++ main/sched.c	(working copy)
@@ -357,6 +357,10 @@
 {
 	struct timeval now = ast_tvnow();
 
+	if (when < 0) {
+		ast_log(LOG_WARNING, "negative time interval %d (interpreted as %u ms) please report this to the Asterisk team!\n", when, when);
+	}
+
 	/*ast_debug(1, "TV -> %lu,%lu\n", tv->tv_sec, tv->tv_usec);*/
 	if (ast_tvzero(*t))	/* not supplied, default to now */
 		*t = now;