[Serusers] HELP!!! User AGent don't send ACK after 200 OK message.! HELP!!!

Hi! I have a very weird trouble here... I've been watching using a sniffer the messages exchange between 2 UA and SER .... and I can see very clear that the Caller UA never reply with an ACK message after it has received the 200 OK message from the Callee UA. It just open the rtp port and start sending out rtp streams... I have tried this either using ATA and Sipuras, but always the same result... because of this the caller never received audio streams from the callee due this last one is waiting for the ACK message after it sent out the 200 OK message... I have no clue why they are no sending this message to the callee.. I need help please!!! Any help or clue or hint...or whatever will be really appreciate!!! Thanks in advance! Armando Marrero Cti, Miami, FL