First, I would like to thank for all advices provided here. Now we can detect the call accepted by sending a character to the transmitter to notify the call accepted. In this part, it is no problem. However, after the call we try to send the data from memory via MT8888 to another end which is also MT8888. Another problem is happened. They can't send/receive the data because of the time problem although we try to switch between send and receive to maintain synchronization. Any advices ?