moh issue with mm-same-ip disabled

Hi guys

another strange problem has occured me yesterday and maybe can be usefull for someone else .

Scenario is an access-backbone scenario with public EP behind nat , i've configured public realm(shared among all the EP) and core realm with those settings for media :

mm-in-realm enabled
mm-in-network enabled
mm-same-ip disabled
mm-in-system enabled

 

now the idea is to release media beetween EP behind the same nat and this is accomplished without issue, the problem is that when i try to start MOH , it works only when i push the button on the phone the second time , but not at the first time (no audio) .