[SR-Users] How to drop negative final response and send ACK

Hi, I have a problem with the following scenario: I have web and mobile sip clients for the same username. If both clients are idle, INVITE is forked and both clients ring. If web client is busy and mobile client is online and registered, the mobile client rings and the caller hears ringback tone. If web client is busy and mobile client is offline, before the mobile client receives push notification and registers, web client sends 486 busy response and caller hears busy tone. In this last scenario, I want to wait some time for the mobile client to register and ring. I tried to drop 486 response in onreply_route[x], but I found that final responses cannot be dropped in this route. I dropped 486 in reply_route, but I couldn't send an ACK for this transaction. Is there a way to implement this scenario? I would appreciate if you have any suggestions. Thanks, Koray