Asterisk Calls ACD problem

Dear , We have a running asterisk to asterisk calls service. we run now VPN on both server with openvpn and send calls from one server to another with IAX2 and codec Speex. Calls running fine but, we face calls ACD issue . 90% calls drop in 1 minute. while we dial manually its working fine but, while we send live calls most of calls connect and disconnect in 20/25 sec. Need to fix it immediately. need a real asterisk expert. waiting for update. regards, MHASAN