[Users] OpenSER + Asterisk - Music On Hold

Hi all, I have an OpenSER server in front of serveral Asterisk acting as a load balancer and registrar server. We're offering both, inbound and outbound call services. When an outbound call is made, OpenSER, through the dispatcher module, choose an Asterisk server to handle the media of the call. When an inbound call is received (by a PSTN GW interconnected with one of the Asterisks), Asterisk calls SIP/username@openser. Media flows directly from user to Asterisks without using RTPProxy as every Asterisk server has got a public IP Address.. I have the following problem with MOH. If a user tries to put on hold an outbound call (placed by him) everything is OK, Asterisk start playing MOH and stops when the user wants to stop it. But, if a user wants to put on hold an inbound call (a call just answered), as soon as it press the hold button another call to the caller is originated and the first call is not put on hold by the Asterisk I guess the problem is that, in this case, the asterisk doesn't recognise the INVITE as a re-INVITE and originate a new call instead of putting the other on hold. Do you have any idea on how to solve the problem ? Every suggestion is appreciated. Regards Edoardo Serra