Context 'macro-dialout-trunk-predial-hook' for macro 'dialout-trunk-predial-hook' lacks 's' extension, priority

Here is another warning message that we get in our logs, this one appears rather frequently:

WARNING[19401] app_macro.c: Context 'macro-dialout-trunk-predial-hook' for macro 'dialout-trunk-predial-hook' lacks 's' extension, priority 1

Any ideas what its about and how to stop it from appearing?