Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 258776)
+++ channels/chan_sip.c	(working copy)
@@ -6147,7 +6147,9 @@
 	return f;
 }
 
-/*! \brief Read SIP RTP from channel */
+/*! \brief Read SIP RTP from channel
+ * Called with ast locked
+ */
 static struct ast_frame *sip_read(struct ast_channel *ast)
 {
 	struct ast_frame *fr;
@@ -6156,14 +6158,19 @@
 
 	sip_pvt_lock(p);
 	fr = sip_rtp_read(ast, p, &faxdetected);
+
+	if (fr == &ast_eagain_frame) {
+		/* Don't update the timeout counter, because we haven't actually received a frame */
+		sip_pvt_unlock(p);
+		return fr;
+	}
+
 	p->lastrtprx = time(NULL);
 
 	/* If we detect a CNG tone and fax detection is enabled then send us off to the fax extension */
 	if (faxdetected && ast_test_flag(&p->flags[1], SIP_PAGE2_FAX_DETECT)) {
-		ast_channel_lock(ast);
 		if (strcmp(ast->exten, "fax")) {
 			const char *target_context = S_OR(ast->macrocontext, ast->context);
-			ast_channel_unlock(ast);
 			if (ast_exists_extension(ast, target_context, "fax", 1, ast->cid.cid_num)) {
 				ast_verbose(VERBOSE_PREFIX_2 "Redirecting '%s' to fax extension\n", ast->name);
 				pbx_builtin_setvar_helper(ast, "FAXEXTEN", ast->exten);
@@ -6173,11 +6180,9 @@
 				fr = &ast_null_frame;
 			} else {
 				ast_log(LOG_NOTICE, "Fax detected but no fax extension\n");
-                        }
-		} else {
-			ast_channel_unlock(ast);
-                }
-        }
+			}
+		}
+	}
 
 	/* Only allow audio through if they sent progress with SDP, or if the channel is actually answered */
 	if (fr && fr->frametype == AST_FRAME_VOICE && p->invitestate != INV_EARLY_MEDIA && ast->_state != AST_STATE_UP) {
Index: include/asterisk/frame.h
===================================================================
--- include/asterisk/frame.h	(revision 258776)
+++ include/asterisk/frame.h	(working copy)
@@ -182,6 +182,7 @@
 /*! Queueing a null frame is fairly common, so we declare a global null frame object
     for this purpose instead of having to declare one on the stack */
 extern struct ast_frame ast_null_frame;
+extern struct ast_frame ast_eagain_frame;
 
 /*! \brief Offset into a frame's data buffer.
  *
Index: main/rtp.c
===================================================================
--- main/rtp.c	(revision 258776)
+++ main/rtp.c	(working copy)
@@ -1456,7 +1456,10 @@
 	
 	/* Cache where the header will go */
 	res = recvfrom(rtp->s, rtp->rawdata + AST_FRIENDLY_OFFSET, sizeof(rtp->rawdata) - AST_FRIENDLY_OFFSET,
-					0, (struct sockaddr *)&sin, &len);
+					MSG_DONTWAIT, (struct sockaddr *)&sin, &len);
+	if (res == -1 && errno == EAGAIN) {
+		return &ast_eagain_frame;
+	}
 
 	/* If strict RTP protection is enabled see if we need to learn this address or if the packet should be dropped */
 	if (rtp->strict_rtp_state == STRICT_RTP_LEARN) {
Index: main/frame.c
===================================================================
--- main/frame.c	(revision 258776)
+++ main/frame.c	(working copy)
@@ -120,6 +120,7 @@
 };
 
 struct ast_frame ast_null_frame = { AST_FRAME_NULL, };
+struct ast_frame ast_eagain_frame = { AST_FRAME_NULL, EAGAIN, };
 
 static int smoother_frame_feed(struct ast_smoother *s, struct ast_frame *f, int swap)
 {