There is a communication issue between the pbx and the phone
The blinking light usually means the phone is "ringing" so it seems the rest of the setup did not complete.
Usually when the call connects a 200 OK message is sent followed by an ACK message being received for that 200 OK. Coupled with 1 way audio I say you have some firewall issues.
The easiest way to find out what is wrong on the softphone is to run wireshark on the computer the softphone is on and see what is missing.