diff -Naur asterisk/channels/chan_zap.c asterisk-patched/channels/chan_zap.c
--- asterisk/channels/chan_zap.c	2005-06-10 00:34:29.000000000 +0200
+++ asterisk-patched/channels/chan_zap.c	2005-06-15 13:44:24.000000000 +0200
@@ -252,6 +252,7 @@
 static int cur_debounce = -1;
 
 static int priindication_oob = 0;
+static int handleprogresscause = 0;
 
 #ifdef ZAPATA_PRI
 static int minunused = 2;
@@ -524,6 +525,7 @@
 	unsigned int echocanon:1;
 	unsigned int faxhandled:1;			/* Has a fax tone already been handled? */
 	unsigned int firstradio:1;
+	unsigned int handleprogresscause:1;		/* Whether we should handle PROGRESS messages with cause code */
 	unsigned int hanguponpolarityswitch:1;
 	unsigned int hidecallerid;
 	unsigned int ignoredtmf:1;
@@ -6844,6 +6846,7 @@
 		tmp->restrictcid = restrictcid;
 		tmp->use_callingpres = use_callingpres;
 		tmp->priindication_oob = priindication_oob;
+		tmp->handleprogresscause = handleprogresscause;
 		tmp->emdigitwait = cur_emdigitwait;
 		if (tmp->usedistinctiveringdetection) {
 			if (!tmp->use_callerid) {
@@ -8301,6 +8304,20 @@
 #endif
 						struct ast_frame f = { AST_FRAME_CONTROL, AST_CONTROL_PROGRESS, };
 						
+						/* handle cause code 'user busy' (17) in q.931 PROGRESS messages*/
+						if (e->proceeding.cause > -1) {
+							if (option_verbose > 2) 
+								ast_verbose(VERBOSE_PREFIX_3 "PROGRESS with cause code %d receiveded!\n",e->proceeding.cause);
+							if ( handleprogresscause && (e->proceeding.cause == AST_CAUSE_USER_BUSY)) {
+								if (pri->pvts[chanpos]->owner) {
+									if (option_verbose > 2) 
+										ast_verbose(VERBOSE_PREFIX_3 "PROGRESS with 'user busy' received, signaling AST_CONTROL_BUSY instead of AST_CONTROL_PROGRESS\n");
+									pri->pvts[chanpos]->owner->hangupcause = e->proceeding.cause;
+									f.subclass = AST_CONTROL_BUSY;
+								}
+							}
+						}
+
 						ast_mutex_lock(&pri->pvts[chanpos]->lock);
 						ast_log(LOG_DEBUG, "Queuing frame from PRI_EVENT_PROGRESS on channel %d/%d span %d\n",
 								pri->pvts[chanpos]->logicalspan, pri->pvts[chanpos]->prioffset,pri->span);
@@ -10280,6 +10297,13 @@
 				else
 					ast_log(LOG_WARNING, "'%s' is not a valid pri indication value, should be 'inband' or 'outofband' at line %d\n",
 						v->value, v->lineno);
+			} else if (!strcasecmp(v->name, "handleprogresscause")) {
+				if (!strcasecmp(v->value, "yes"))
+					handleprogresscause = 1;
+				else if (!strcasecmp(v->value, "no"))
+					handleprogresscause = 0;
+				else
+					ast_log(LOG_WARNING, "'%s' is not a valid value for 'handleprogresscause', should be 'yes' or 'no' at line %d\n",v->value, v->lineno);
 			} else if (!strcasecmp(v->name, "internationalprefix")) {
 				ast_copy_string(internationalprefix, v->value, sizeof(internationalprefix));
 			} else if (!strcasecmp(v->name, "nationalprefix")) {
diff -Naur asterisk/configs/zapata.conf.sample asterisk-patched/configs/zapata.conf.sample
--- asterisk/configs/zapata.conf.sample	2005-05-12 17:04:55.000000000 +0200
+++ asterisk-patched/configs/zapata.conf.sample	2005-06-15 13:25:24.000000000 +0200
@@ -124,6 +124,15 @@
 ;
 ; priindication = outofband
 ;
+; Handle cause code in PROGRESS message
+; Some Telcos send PROGRESS messages with a cause code to signal
+; certain line conditions, e.g. user busy.
+; Turn this option on to enable busy detection in PROGRESS messages.
+; Allowed values: yes/no
+; Default = no
+;
+;handleprogresscause=yes
+;
 ; ISDN Timers
 ; All of the ISDN timers and counters that are used are configurable.  Specify 
 ; the timer name, and its value (in ms for timers)