diff --git a/bin/check.sh b/bin/check.sh
index 376447c7..4ceafe97 100755
--- a/bin/check.sh
+++ b/bin/check.sh
@@ -396,9 +396,9 @@ function run_sipp
     responder_pid="${responder_pid} ${base}:${pid}"
 
     if [ "${foreign_dom}" == "no" ]; then
-      PORT=$(check_port "${PORT}")
+      PORT=$(check_port "$((PORT+1))")
     fi
-    MPORT=$(check_mport "${MPORT}")
+    MPORT=$(check_mport "$((MPORT+2))")
   done
 
   local status=0