Audio does not pass until DTMF is sent

I have Asterisk set up and have assigned two extensions (one using a soft phone and the other with an Avaya desk phone). When the other person calls my extension, I can hear them, but they can’t hear me until I press one of the DTMF buttons, then the audio path is opened and conversation is normal.

I have all of the required ports open on the firewall.

Any ideas?