diff --git a/lib/redmine/helpers/url.rb b/lib/redmine/helpers/url.rb
index af4854c8d..a70aea022 100644
--- a/lib/redmine/helpers/url.rb
+++ b/lib/redmine/helpers/url.rb
@@ -23,7 +23,7 @@ module Redmine
   module Helpers
     module URL
       # safe for resources fetched without user interaction?
-      def uri_with_safe_scheme?(uri, schemes = ['http', 'https', 'ftp', 'mailto', nil])
+      def uri_with_safe_scheme?(uri, schemes = ['http', 'https', 'ftp', 'mailto', 'tel', nil])
         # URLs relative to the current document or document root (without a protocol
         # separator, should be harmless
         return true unless uri.to_s.include? ":"